This is the MySQL reference manual; it documents MySQL version 3.23.6-alpha.
MySQL is a very fast, multi-threaded, multi-user and robust SQL (Structured Query Language) database server.
For Unix and OS/2 platforms, MySQL is basically free; for Microsoft platforms you must get a MySQL license after a trial time of 30 days. See section 3 MySQL licensing and support.
The MySQL home page provides the latest information about MySQL.
For a discussion of MySQL's capabilities, see section 1.4 The main features of MySQL.
For installation instructions, see section 4 Installing MySQL. For tips on porting MySQL to new machines or operating systems, see section G Comments on porting to other systems.
For information about upgrading from a 3.21 release, see section 4.16.2 Upgrading from a 3.21 version to 3.22.
For a tutorial introduction to MySQL, see section 8 MySQL Tutorial.
For examples of SQL and benchmarking information, see the benchmarking directory (`sql-bench' in the distribution).
For a history of new features and bug fixes, see section D MySQL change history.
For a list of currently known bugs and misfeatures, see section E Known errors and design deficiencies in MySQL.
For future plans, see section F List of things we want to add to MySQL in the future (The TODO).
For a list of all the contributors to this project, see section C Contributors to MySQL.
IMPORTANT:
Send bug (error) reports, questions and comments to the mailing list at
For source distributions, the mysqlbug
script can be found in the
`scripts' directory. For binary distributions, mysqlbug
can
be found in the `bin' directory.
If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at (docs@mysql.com).
MySQL is a true multi-user, multi-threaded SQL database
server. SQL (Structured Query Language) is the most popular and
standardized database language in the world. MySQL is a
client/server implementation that consists of a server daemon
mysqld
and many different client programs and libraries.
SQL is a standardized language that makes it easy to store, update and access information. For example, you can use SQL to retrieve product information and store customer information for a web site. MySQL is also fast and flexible enough to allow you to store logs and pictures in it.
The main goals of MySQL are speed, robustness and ease of use. MySQL was originally developed because we needed a SQL server that could handle very large databases an order of magnitude faster than what any database vendor could offer to us on inexpensive hardware. We have now been using MySQL since 1996 in an environment with more than 40 databases containing 10,000 tables, of which more than 500 have more than 7 million rows. This is about 100 gigabytes of mission-critical data.
The base upon which MySQL is built is a set of routines that have been used in a highly demanding production environment for many years. Although MySQL is still under development, it already offers a rich and highly useful function set.
The official way to pronounce MySQL is ``My Ess Que Ell'' (Not MY-SEQUEL).
This manual is currently available in Texinfo, plain text, Info, HTML, PostScript and PDF versions. Because of their size, PostScript and PDF versions are not included with the main MySQL distribution, but are available for separate download at http://www.mysql.com.
The primary document is the Texinfo file. The HTML version is produced
automatically with a modified version of texi2html
. The plain
text and Info versions are produced with makeinfo
. The Postscript
version is produced using texi2dvi
and dvips
. The PDF
version is produced with pdftex
.
This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Paul DuBois and Kim Aldale. For other contributors, see section C Contributors to MySQL.
This manual uses certain typographical conventions:
constant
mysqladmin
works, invoke it with the
--help
option.''
When commands are shown that are meant to be executed by a particular
program, the program is indicated by the prompt shown with the command. For
example, shell>
indicates a command that you execute from your login
shell, and mysql>
indicates a command that you execute from the
mysql
client:
shell> type a shell command here mysql> type a mysql command here
Shell commands are shown using Bourne shell syntax. If you are using a
csh
-style shell, you may need to issue commands slightly differently.
For example, the sequence to set an environment variable and run a command
looks like this in Bourne shell syntax:
shell> VARNAME=value some_command
For csh
, you would execute the sequence like this:
shell> setenv VARNAME value shell> some_command
Database, table and column names often must be substituted into commands. To
indicate that such substitution is necessary, this manual uses
db_name
, tbl_name
and col_name
. For example, you might
see a statement like this:
mysql> SELECT col_name FROM db_name.tbl_name;
This means that if you were to enter a similar statement, you would supply your own database, table and column names, perhaps like this:
mysql> SELECT author_name FROM biblio_db.author_list;
SQL statements may be written in uppercase or lowercase. When this manual
shows a SQL statement, uppercase is used for particular keywords if those
keywords are under discussion (to emphasize them) and lowercase is used for
the rest of the statement. So you might see the following in a discussion of
the SELECT
statement:
mysql> SELECT count(*) FROM tbl_name;
On the other hand, in a discussion of the COUNT()
function, the
statement would be written like this:
mysql> select COUNT(*) from tbl_name;
If no particular emphasis is intended, all keywords are written uniformly in uppercase.
In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses:
DROP TABLE [IF EXISTS] tbl_name
When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets. When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str) {DESCRIBE | DESC} tbl_name {col_name | wild}
We once started off with the intention of using mSQL
to connect to our
tables using our own fast low-level (ISAM) routines. However, after some
testing we came to the conclusion that mSQL
was not fast enough or
flexible enough for our needs. This resulted in a new SQL interface to our
database but with almost the same API interface as mSQL
. This API was
chosen to ease porting of third-party code.
The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, Monty's daughter (some years younger) is also named My. So which of the two gave its name to MySQL is still a mystery, even for us.
The following list describes some of the important characteristics of MySQL:
FLOAT
, DOUBLE
, CHAR
, VARCHAR
,
TEXT
, BLOB
, DATE
, TIME
, DATETIME
,
TIMESTAMP
, YEAR
, SET
and ENUM
types. See section 7.3 Column types.
SELECT
and WHERE
parts of queries. Example:
mysql> SELECT CONCAT(first_name, " ", last_name) FROM tbl_name WHERE income/dependents > 10000 AND age > 30;
GROUP BY
and ORDER BY
clauses. Support for group functions (COUNT()
,
COUNT(DISTINCT)
, AVG()
, STD()
, SUM()
,
MAX()
and MIN()
).
LEFT OUTER JOIN
with ANSI SQL and ODBC syntax.
CHAR
or VARCHAR
field.
INSERT
to insert a
subset of a table's columns; those columns that are not explicitly given
values are set to their default values.
libtool
for portability.
purify
).
myisamchk
, a very fast utility for table checking, optimization
and repair.
See section 13 Maintaining a MySQL installation.
DELETE
, INSERT
, REPLACE
, and UPDATE
return
how many rows were changed (affected).
ABS
is a valid column name. The only restriction is that for a function call, no
spaces are allowed between the function name and the `(' that follows it.
See section 7.31 Is MySQL picky about reserved words?.
--help
or -?
options to obtain online assistance.
SHOW
command can be used to retrieve
information about databases, tables and indexes. The EXPLAIN
command
can be used to determine how the optimizer resolves a query.
This section addresses the questions, ``how stable is MySQL?'' and, ``can I depend on MySQL in this project?'' Here we will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).
At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of ``untested code'' that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features), and we hope that it will be possible to label one of the next releases ``stable''.
Each release of MySQL has been usable and there have been problems only when users start to use code from ``the gray zones''. Naturally, outside users can't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with the 3.22.x version of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are ``design''-related. See section E Known errors and design deficiencies in MySQL.
MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is:
mysql
, mysqladmin
and mysqlshow
,
mysqldump
, and mysqlimport
.
fcntl()
). In these cases, you should run the
MySQL daemon with the --skip-locking
flag. Problems are known
to occur on some Linux systems and on SunOS when using NFS-mounted file
systems.
fcntl()
call, which is fixed
by using the --skip-locking
option to mysqld
. Some people
have reported lockup problems with the 0.5 release.
SELECT
statements are usually done in one time frame so there shouldn't be a mutex
locking/thread juggling.
LOAD DATA ...
, INSERT ... SELECT
-- Stable
ALTER TABLE
-- Stable
mysqlaccess
-- Stable
GRANT
-- Gamma
TcX provides email support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.
MySQL itself has no problems with Year 2000 (Y2K) compliance:
2069
; all 2-digit years are regarded to be in the range
1970
to 2069
, which means that if you store 01
in a
year
column, MySQL treats it as 2001
.
YEAR
column type
can store years 0
and 1901
to 2155
in 1 byte and display
them using 2 or 4 digits.
You may run into problems with applications that use MySQL in a
way that is not Y2K-safe. For example, many old applications store
or manipulate years using 2-digit values (which are ambiguous) rather than
4-digit values. This problem may be compounded by applications that use
values such as 00
or 99
as ``missing'' value indicators.
Unfortunately, these problems may be difficult to fix, since different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.
Here is a simple demonstration illustrating that MySQL doesn't have any problems with dates until the year 2030!
mysql> DROP TABLE IF EXISTS y2k; mysql> CREATE TABLE y2k (date date, date_time datetime, time_stamp timestamp); mysql> INSERT INTO y2k VALUES ("1998-12-31","1998-12-31 23:59:59",19981231235959); mysql> INSERT INTO y2k VALUES ("1999-01-01","1999-01-01 00:00:00",19990101000000); mysql> INSERT INTO y2k VALUES ("1999-09-09","1999-09-09 23:59:59",19990909235959); mysql> INSERT INTO y2k VALUES ("2000-01-01","2000-01-01 00:00:00",20000101000000); mysql> INSERT INTO y2k VALUES ("2000-02-28","2000-02-28 00:00:00",20000228000000); mysql> INSERT INTO y2k VALUES ("2000-02-29","2000-02-29 00:00:00",20000229000000); mysql> INSERT INTO y2k VALUES ("2000-03-01","2000-03-01 00:00:00",20000301000000); mysql> INSERT INTO y2k VALUES ("2000-12-31","2000-12-31 23:59:59",20001231235959); mysql> INSERT INTO y2k VALUES ("2001-01-01","2001-01-01 00:00:00",20010101000000); mysql> INSERT INTO y2k VALUES ("2004-12-31","2004-12-31 23:59:59",20041231235959); mysql> INSERT INTO y2k VALUES ("2005-01-01","2005-01-01 00:00:00",20050101000000); mysql> INSERT INTO y2k VALUES ("2030-01-01","2030-01-01 00:00:00",20300101000000); mysql> INSERT INTO y2k VALUES ("2050-01-01","2050-01-01 00:00:00",20500101000000); mysql> SELECT * FROM y2k; +------------+---------------------+----------------+ | date | date_time | time_stamp | +------------+---------------------+----------------+ | 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 | | 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 | | 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 | | 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 | | 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 | | 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 | | 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 | | 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 | | 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 | | 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 | | 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 | | 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 | | 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 | +------------+---------------------+----------------+ 13 rows in set (0.00 sec)
This shows that the DATE
and DATETIME
types are will not
give any problems with future dates (they handle dates until the year
9999).
The TIMESTAMP
type, that is used to store the current time, has a
range up to only 2030-01-01
. TIMESTAMP
has a range of
1970
to 2030
on 32-bit machines (signed value). On 64-bit
machines it handles times up to 2106
(unsigned value).
Even though MySQL is Y2K-compliant, it is your responsibility to provide unambiguous input. See section 7.3.6.1 Y2K issues and date types for MySQL's rules for dealing with ambiguous date input data (data containing 2-digit year values).
This book has been recommended by a several people on the MySQL mailing list:
Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky The Practical SQL Handbook: Using Structured Query Language Second Edition Addison-Wesley ISBN 0-201-62623-3 http://www.awl.com
This book has also received some recommendations by MySQL users:
Martin Gruber Understanding SQL ISBN 0-89588-644-8 Publisher Sybex 510 523 8233 Alameda, CA USA
A SQL tutorial is available on the net at http://www.geocities.com/SiliconValley/Vista/2207/sql1.html
SQL in 21 Tagen (online book in German language): http://www.mut.de/leseecke/buecher/sql/inhalt.htm
MySQL
with other programsmSQL
mSQL
TCL
DBI
/DBD
.
DBI
/DBD
modules homepage
There are also many web pages that use MySQL. See section A Some MySQL users. Send any additions to this list to MySQL logo somewhere (It is okay to have it on a ``used tools'' page or something similar) to be added.
To subscribe to the main MySQL mailing list, send a message to the electronic mail address mysql-subscribe@lists.mysql.com.
To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address mysql-unsubscribe@lists.mysql.com.
Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.
If your reply address is not valid, you can specify your address explicitly.
Adding a hyphen to the subscribe or unsubscribe command word, followed by
your address with the `@' character in your address replaced by a
`='. For example, to subscribe john@host.domain
, send a message
to mysql-subscribe-john=host.domain@lists.mysql.com
.
Mail to mysql-subscribe@lists.mysql.com or ezmlm mailing list processor. Information about ezmlm is available at the ezmlm Website.
To post a message to the list itself, send your message to
mysql@lists.mysql.com
. However, please do not send mail about
subscribing or unsubscribing to mysql@lists.mysql.com, since any
mail sent to that address is distributed automatically to thousands of other
users.
Your local site may have many subscribers to mysql@lists.mysql.com.
If so, it may have a local mailing list, so that messages sent from
lists.mysql.com
to your site are propagated to the local list. In such
cases, please contact your system administrator to be added to or dropped
from the local MySQL list.
The following MySQL mailing lists exist:
announce
mysql
mysql-digest
mysql
list in digest form. That means you get all individual
messages, sent as one large mail message once a day.
java
java-digest
java
list.
win32
win32-digest
win32
list.
myodbc
myodbc-digest
myodbc
list.
msql-mysql-modules
msql-mysql-modules-digest
msql-mysql-modules
list.
developer
developer-digest
developer
list.
You subscribe or unsubscribe to all lists in the same way as described
above. In your subscribe or unsubscribe message, just put the appropriate
mailing list name rather than mysql
. For example, to subscribe to or
unsubscribe from the myodbc
list, send a message to
Before posting a bug report or question, please do the following:
http://www.mysql.com/Manual_chapter/manual_toc.htmlWe try to keep the manual up to date by updating it frequently with solutions to newly found problems!
http://www.mysql.com/doc.html
If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to
Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.
We encourage everyone to use the mysqlbug
script to generate a bug
report (or a report about any problem), if possible. mysqlbug
can be
found in the `scripts' directory in the source distribution, or, for a
binary distribution, in the `bin' directory under your MySQL
installation directory. If you are unable to use mysqlbug
, you should
still include all the necessary information listed in this section.
The mysqlbug
script helps you generate a report by determining much
of the following information automatically, but if something important is
missing, please include it with your message! Please read this section
carefully and make sure that all the information described here is included
in your report.
Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.
The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have MySQL installed on (including the platform version number). This is highly relevant information and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like ``Why doesn't this work for me?'' and then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.
Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL related. Most compilers are under development all the time and become better version by version, too. To determine whether or not your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.
It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem.
If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case sensitivity should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!
If you have a problem with MyODBC, you should try to genereate a MyODBC trace file. See section 16.6 Reporting problems with MyODBC.
Please remember that many of the people who will read your report will
do so using an 80-column display. When generating reports or examples
using the mysql
command line tool, you should therefore use
the --vertical
option (or the \G
statement terminator)
for output which would exceed the available width for such a display
(for example, with the EXPLAIN SELECT
statement; see the
example below).
Please include the following information in your report:
mysqladmin version
. mysqladmin
can be
found in the `bin' directory under your MySQL installation
directory.
uname -a
.
mysqldump --no-data db_name tbl_name1 tbl_name2 ...
This is very easy
to do and is a powerful way to get information about any table in a database
that will help us create a situation matching the one you have.
SELECT
statements, you should
always include the output of EXPLAIN SELECT ...
, and at least the
number of rows that the SELECT
statement produces. The more
information you give about your situation, the more likely it is that someone
can help you! For example, the following is an example of a very good bug
report (it should of course be posted with the mysqlbug
script):
Example run using the mysql
command line tool (note the use of the
\G
statement terminator for statements whose output width would
otherwise exceed that of an 80-column display device):
mysql> SHOW VARIABLES; mysql> SHOW COLUMNS FROM ...\G <output-from-SHOW-COLUMNS> mysql> EXPLAIN SELECT ...\G <output-from-EXPLAIN> mysql> FLUSH STATUS; mysql> SELECT ...; <A short version of the output from SELECT, including the time taken to run the query> mysql> SHOW STATUS; <output from SHOW STATUS>
mysqladmin variables extended-status processlist
in your mail to
provide some information of how your system is performing!
ftp
to
transfer it to ftp://www.mysql.com/pub/mysql/secret/. If the data are
really top secret and you don't want to show them even to us, then go ahead
and provide an example using other names, but please regard this as the last
choice.
mysqld
daemon and that you use to run any MySQL client programs. The
options to programs like mysqld
and mysql
, and to the
configure
script are often keys to answers and very relevant! It is
never a bad idea to include them anyway! If you use any modules, such as
Perl or PHP, please include the version number(s) of those as well.
mysqldump
and create a `README' file
that describes your problem.
Create a compressed archive of your files using
tar
and gzip
or zip
, and use ftp
to transfer the archive to ftp://www.mysql.com/pub/mysql/secret/.
Then send a short description of the problem to mysql@lists.mysql.com.
mysqlaccess
, the output of mysqladmin reload
and all
the error messages you get when trying to connect! When you test your
privileges, you should first run mysqlaccess
. After this, execute
mysqladmin reload version
, and last you should try to connect with the
program that gives you trouble. mysqlaccess
can be found in the
`bin' directory under your MySQL installation directory.
parse error
, please check your syntax closely! If
you can't find something wrong with it, it's extremely likely that your
current version of MySQL doesn't support the query you are
using. If you are using the current version and the manual at
http://www.mysql.com/doc.html doesn't cover the syntax you are
using, MySQL doesn't support your query. In this case, your
only options are to implement the syntax yourself or email
If the manual covers the syntax you are using, but you have an older version
of MySQL, you should check the MySQL change history to see
when the syntax was implemented. See section D MySQL change history. In this case, you have the
option of upgrading to a newer version of MySQL.
myisamchk
.
See section 13 Maintaining a MySQL installation.
mysqld
should
NEVER crash a table if nothing killed it in the middle of an
update! If you can find the source of why mysqld
dies, it's much
easier for us to provide you with a fix for the problem!
If you are a support customer, please cross-post the bug report to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.
For information on reporting bugs in MyODBC, see section 16.2 How to report problems with MyODBC.
For solutions to some common problems, see See section 18 Problems and common errors.
When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!
If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.
Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.
Please don't post mail messages from your browser with HTML mode turned on! Many users doesn't read mail with a browser!
This chapter describes MySQL licensing and support arrangements, including:
The formal terms of the license for non-Microsoft operating systems such as Unix or OS/2 are specified in section J The MySQL server license for non Microsoft operating systems. Basically, our licensing policy is as follows:
mysqld
server. However, a
multiple-CPU machine counts as a single machine, and there is no restriction
on the number of MySQL servers that run on one machine, or on the
number of clients concurrently connected to a server running on that
machine!
mysql
command line client includes code from the readline
library that is under the GNU Public License.
myisampack
utility for creating fast compressed
read-only databases. (The server includes support for reading such
databases but not the packing tool used to create them.) When support
agreements generate sufficient revenue, we will release this tool under
the same license as the MySQL server.
For use under Microsoft operating systems (Win95/Win98/WinNT), you need a MySQL license after a trial period of 30 days, with the exception that licenses may be obtained upon request at no cost for educational use or for university- or government-sponsored research settings. See section K The MySQL license for Microsoft operating systems. A shareware version of MySQL-Win32 that you can try before buying is available at http://www.mysql.com/mysql_w32.htmy. After you have paid, you will get a password that will enable you to access the newest MySQL-Win32 version.
If you have any questions as to whether or not a license is required for your particular use of MySQL, please contact us. See section 3.5.2 Contact information.
If you require a MySQL license, the easiest way to pay for it is to use the license form at TcX's secure server at https://www.mysql.com/license.htmy. Other forms of payment are discussed in section 3.5.1 Payment information.
There are several different copyrights on the MySQL distribution:
mysqlclient
library and programs in the `client' directory is in the public
domain. Each file that is in the public domain has a header which clearly
states so. This includes everything in the `client' directory and some parts of
the mysys
, mystring
and dbug
libraries.
getopt
) are covered by the
``GNU LIBRARY LIBRARY GENERAL PUBLIC LICENSE''. See the
`mysys/COPYING.LIB' file.
readline
) are covered by the
``GNU GENERAL PUBLIC LICENSE''. See the `readline/COPYING' file.
regexp
library) are covered by a Berkeley
style copyright.
The following points set forth the philosophy behind our copyright policy:
We may choose to distribute older versions of MySQL with the GPL in the future. However, these versions will be identified as GNU MySQL. Also, all copyright notices in the relevant files will be changed to the GPL.
This section is a clarification of the license terms that are set forth in the ``MySQL FREE PUBLIC LICENSE'' (FPL). See section J The MySQL server license for non Microsoft operating systems.
MySQL may be used freely, including by commercial entities for evaluation or unsupported internal use. However, distribution for commercial purposes of MySQL, or anything containing or derived from MySQL in whole or in part, requires a written commercial license from TcX AB, the sole entity authorized to grant such licenses.
You may not include MySQL ``free'' in a package containing anything for which a charge is being made, except as noted below.
The intent of the exception provided in the second clause of the license is to allow commercial organizations operating an FTP server or a bulletin board to distribute MySQL freely from it, provided that:
If you want to distribute software in a commercial context that incorporates MySQL and you do not want to meet these conditions, you should contact TcX AB to find out about commercial licensing, which involves a payment. The only ways you legally can distribute MySQL or anything containing MySQL are by distributing MySQL under the requirements of the FPL, or by getting a commercial license from TcX AB.
This section describes some situations illustrating whether or not you must license the MySQL server. Generally these examples involve providing MySQL as part of a product or service that you are selling to a customer, or requiring that MySQL be used in conjunction with your product. In such cases, it is your responsibility to obtain a license for the customer if one is necessary. (This requirement is waived if your customer already has a MySQL license. But the seller must send customer information and the license number to TcX, and the license must be a full license, not an OEM license.)
Note that a single MySQL license covers any number of
CPUs/users/customers/mysqld
servers on a machine!
To determine whether or not you need a MySQL license when selling your application, you should ask whether the proper functioning of your application is contingent on the use of MySQL and whether you include MySQL with your product. There are several cases to consider:
mysqld
server. For example, if you've designed your
application around MySQL, then you've really made a commercial
product that requires the engine, so you need a license.
If your application does not require MySQL, you need not obtain a
license. For example, if MySQL just added some new optional
features to your product (such as adding logging to a database if
MySQL is used rather than logging to a text file), it should fall
within normal use, and a license would not be required.
In other words, you need a license if you sell a product designed
specifically for use with MySQL or that requires the MySQL
server to function at all. This is true whether or not you provide
MySQL for your client as part of your product distribution.
It also depends on what you're doing for the client. Do you plan to provide
your client with detailed instructions on installing MySQL with your
software? Then your product may be contingent on the use of MySQL;
if so, you need to buy a license. If you are simply tying into a database
that you expect already to have been installed by the time your software is
purchased, then you probably don't need a license.
mysqld
server,
because in this case you are selling a system that includes MySQL.
This is true whether the use of MySQL with your product
is required or optional.
If you perform MySQL installation on a client's machine and any money changes hands for the service (directly or indirectly), then you must buy a MySQL license.
If you sell an application for which MySQL is not strictly required but can be used, a license may be indicated, depending on how MySQL is set up. Suppose your product neither requires MySQL nor includes it in your product distribution, but can be configured to use MySQL for those customers who so desire. (This would be the case, for example, if your product can use any of a number of database engines.)
If the customer obtains and installs MySQL, no license is needed. If you perform that service for your customer, then a license is needed because then you are selling a service that includes MySQL.
Internet Service Providers (ISPs) often host MySQL servers for their customers.
If you are an ISP that allows customers to install and administer MySQL for themselves on your machine with no assistance from you, neither you nor your customer need a MySQL license.
If you charge for MySQL installation and administrative support as part of your customer service, then you need a license because you are selling a service that includes MySQL.
If you use MySQL in conjunction with a web server, you don't have to pay for a license.
This is true even if you run a commercial web server that uses MySQL, since you are not selling MySQL itself. However, in this case we would like you to purchase MySQL support, because MySQL is helping your enterprise.
Our current license prices are shown below. All prices are in US Dollars. If you pay by credit card, the currency is EURO (European Union Euro) so the prices will differ slightly.
Number of licenses | Price per copy | Total |
1 | US $200 | US $200 |
10 pack | US $150 | US $1500 |
50 pack | US $120 | US $6000 |
For high volume (OEM) purchases, the following prices apply:
Number of licenses | Price per copy | Minimum at one time | Minimum payment |
100-999 | US $40 | 100 | US $4000 |
1000-2499 | US $25 | 200 | US $5000 |
2500-4999 | US $20 | 400 | US $8000 |
For OEM purchases, you must act as the middle-man for eventual problems or extension requests from your users. We also require that OEM customers have at least an extended email support contract.
If you have a low-margin high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market and any other information that may be relevant.
After buying 10 MySQL licenses, you will get a personal copy of
the myisampack
utility. You are not allowed to redistribute this
utility but you can distribute tables packed with it.
A full-price license is not a support agreement and includes very minimal support. This means that we try to answer any relevant question. If the answer is in the documentation, we will direct you to the appropriate section. If you have not purchased a license or support, we probably will not answer at all.
If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release.
More comprehensive support is sold separately. Descriptions of what each level of support includes are given in section 3.6 Types of commercial support. Costs for the various types of commercial support are shown below. Support level prices are in EURO (European Union Euro). One EURO is about 1.17 USD.
Type of support | Cost per year |
Basic email support | EURO 170 |
Extended email support | EURO 1000 |
Login support | EURO 2000 |
Extended login support | EURO 5000 |
You may upgrade from any lower level of support to a higher level of support for the difference between the prices of the two support levels.
Currently we can take SWIFT payments, cheques or credit cards.
Payment should be made to:
Postgirot Bank AB 105 06 STOCKHOLM, SWEDEN TCX DataKonsult AB BOX 6434 11382 STOCKHOLM, SWEDEN SWIFT address: PGSI SESS Account number: 96 77 06 - 3
Specify: license and/or support and your name and email address.
In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account.
If you want to pay by cheque, make it payable to ``Monty Program KB'' and mail it to the address below:
TCX DataKonsult AB BOX 6434, Torsgatan 21 11382 STOCKHOLM, SWEDEN
If you want to pay by credit card over the Internet, you can use TcX's secure license form.
You can also print a copy of the license form, fill it in and send it by fax to:
+46-8-729 69 05
If you want us to bill you, you can use the license form and write ``bill us'' in the comment field. You can also mail a message to with your company information and ask us to bill you.
For commercial licensing, or if you have any questions about any of the information in this section, please contact the MySQL licensing team. The much preferred method is by E-Mail to these may take much longer (Fax +46-8-729 69 05).
David Axmark Detron HB Kungsgatan 65 B 753 21 UPPSALA SWEDEN Voice Phone +46-18-10 22 80 (Timezone GMT+1. Swedish and English spoken)
Basic email support is a very inexpensive support option and should be thought of more as a way to support our development of MySQL than as a real support option.
At this support level, the MySQL mailing lists are the preferred means of communication. Questions normally should be mailed to the primary mailing list (mysql@lists.mysql.com) or one of the other regular lists (for example, mysql-win32@lists.mysql.com for Windows-related MySQL questions), as someone else already may have experienced and solved the problem you have. See section 2.2 Asking questions or reporting bugs.
However, by purchasing basic email support, you also have access to the support address mysql-support@mysql.com, which is not available as part of the minimal support that you get by purchasing a MySQL license. This means that for especially critical questions, you can cross-post your message to mysql-support@mysql.com. (If the message contains sensitive data, you should post only to mysql-support@mysql.com.)
REMEMBER! to ALWAYS include your registration number and expiration date when you send a message to
Basic email support includes the following types of service:
Extended email support includes everything in basic email support with these additions:
myisampack
packing tool for
creating fast compressed read-only databases (it does not support
BLOB
or TEXT
types yet). The current server includes
support for reading such databases but not the packing tool used to
create them.
mysqld
for your situation.
Login support includes everything in extended email support with these additions:
kill -9
command).
Extended login support includes everything in login support with these additions:
mysql> select MY_CALCULATION(col_name1,col_name2) from tbl_name;
This chapter describes how to obtain and install MySQL:
Check the MySQL home page for information about the current version and for downloading instructions.
However, the Internet connection at TcX is not so fast; we would prefer that you do the actual downloading from one of the mirror sites listed below.
Please report bad or out of date mirrors to webmaster@mysql.com.
Europe:
North America:
South America:
Asia:
Australia:
Africa:
We use GNU Autoconf so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (versions 2.5 & 2.6) and to a lesser extent on RedHat Linux 5.0.
MySQL has been reported to compile sucessfully on the following operating system/thread package combinations. Note that for many operating systems, the native thread support works only in the latest versions.
glibc
2.0.7
The first decision to make is whether you want to use the latest development release or the last stable release:
crash-me
and
benchmark tests.
See section 10.8 Using your own benchmarks.
The second decision to make is whether you want to use a source distribution or a binary distribution:
The MySQL naming scheme uses release numbers that consist of three
numbers and a suffix. For example, a release name like
mysql-3.21.17-beta
is interpreted like this:
3
) describes the file format. All
version 3 releases have the same file format. When a version 4 appears, every
table will have to be converted to the new format (nice tools for this will
be included, of course).
21
) is the release level. Normally there are two to
choose from. One is the release/stable branch (currently 21
) and the
other is the development branch (currently 22
) . Normally both are
stable, but the development version may have quirks, missing documentation on
new features or may fail to compile on some systems.
17
) is the version number within the
release level. This is incremented for each new distribution. Usually you
want the latest version for the release level you have choosen.
beta
) indicates the stability level of
the release. The possible suffixes are:
alpha
indicates that the release contains some large section of
new code that hasn't been 100% tested. Known bugs (usually there are none)
should be documented in the News section. See section D MySQL change history. There are also new
commands and extensions in most alpha releases.
beta
means that all new code has been tested. No major new features
were added. There should be no known bugs.
gamma
is a beta that has been around a while and seems to work fine.
This is what many other companies call a release.
All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Since the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.
Note that all releases have been tested at least with:
crash-me
test
Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.
MySQL is evolving quite rapidly here at TcX and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.
We also try to help out users who request features that are easy to implement. We also take note of what our licensed users want to have and we especially take note of what our extended email supported customers want and try to help them out.
No one has to download a new release. The News section will tell you if the new release has something you really want. See section D MySQL change history.
We use the following policy when updating MySQL:
The current stable release is 3.22; We have already moved active development to 3.23. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that ``must be done''. ``Somewhat frozen'' means that we may add small things that ``almost surely will not affect anything that's already working''.
This section describes the default layout of the directories created by installing binary and source distributions.
A binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location:
Directory | Contents of directory |
`bin' | Client programs and the mysqld server
|
`data' | Log files, databases |
`include' | Include (header) files |
`lib' | Libraries |
`scripts' | mysql_install_db
|
`share/mysql' | Error message files |
`sql-bench' | Benchmarks |
A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:
Directory | Contents of directory |
`bin' | Client programs and scripts |
`include/mysql' | Include (header) files |
`info' | Documentation in Info format |
`lib/mysql' | Libraries |
`libexec' | The mysqld server
|
`share/mysql' | Error message files |
`sql-bench' | Benchmarks and crash-me test
|
`var' | Databases and log files. |
Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:
mysqld
server is installed in the `libexec'
directory rather than in the `bin' directory.
mysql_install_db
is installed in the `/usr/local/bin' directory
rather than in `/usr/local/mysql/scripts'.
You need the following tools to install a MySQL binary distribution:
gunzip
to uncompress the distribution.
tar
to unpack the distribution. GNU tar
is
known to work.
An alternative installation method under Linux is to use RPM (RedHat Package Manager) distributions. See section 4.6.1 Linux RPM notes.
If you run into problems, PLEASE ALWAYS USE mysqlbug
when
posting questions to mysql@lists.mysql.com. Even if the problem
isn't a bug, mysqlbug
gathers system information that will help others
solve your problem. By not using mysqlbug
, you lessen the likelihood
of getting a solution to your problem! You will find mysqlbug
in the
`bin' directory after you unpack the distribution. See section 2.3 How to report bugs or problems.
The basic commands you must execute to install and use a MySQL binary distribution are:
shell> gunzip < mysql-VERSION-OS.tar.gz | tar xvf - shell> ln -s mysql-VERSION-OS mysql shell> cd mysql shell> scripts/mysql_install_db shell> bin/safe_mysqld &
You can add new users using the bin/mysql_setpermission
script if
you install the DBI
and Msql-Mysql-modules
Perl modules.
Here follows a more detailed description:
To install a binary distribution, follow the steps below, then proceed to section 4.15 Post-installation setup and testing, for post-installation setup and testing:
root
.)
tar
archives and have names like `mysql-VERSION-OS.tar.gz', where
VERSION
is a number (e.g., 3.21.15
), and OS
indicates
the type of operating system for which the distribution is intended (e.g.,
pc-linux-gnu-i586
).
shell> gunzip < mysql-VERSION-OS.tar.gz | tar xvf - shell> ln -s mysql-VERSION-OS mysqlThe first command creates a directory named `mysql-VERSION-OS'. The second command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as `/usr/local/mysql'.
shell> cd mysqlYou will find several files and subdirectories in the
mysql
directory.
The most important for installation purposes are the `bin' and
`scripts' subdirectories.
PATH
environment variable so that your shell finds the MySQL
programs properly.
mysql_install_db
script used to initialize
the server access permissions.
mysqlaccess
and have the MySQL
distribution in some nonstandard place, you must change the location where
mysqlaccess
expects to find the mysql
client. Edit the
`bin/mysqlaccess' script at approximately line 18. Search for a line
that looks like this:
$MYSQL = '/usr/local/bin/mysql'; # path to mysql executableChange the path to reflect the location where
mysql
actually is
stored on your system. If you do not do this, you will get a broken
pipe
error when you run mysqlaccess
.
shell> scripts/mysql_install_dbNote that MySQL versions older than 3.22.10 started the MySQL server when you run
mysql_install_db
. This is no
longer true!
DBI
/DBD
interface,
see section 4.10 Perl installation comments.
support-files/mysql.server
to the location where
your system has its startup files. More information can be found in the
support-files/mysql.server
script itself, and in section 4.15.3 Starting and stopping MySQL automatically.
After everything has been unpacked and installed, you should initialize and test your distribution.
You can start the MySQL server with the following command:
shell> bin/safe_mysqld &
See section 4.15 Post-installation setup and testing.
The recommended way to install MySQL on Linux is by using an RPM
file. The MySQL RPMs are currently being built on a RedHat 5.2
system but should work on other versions of Linux that support rpm
and
use glibc
.
If you have problems with an RPM file, for example Sorry, the host
'xxxx' could not be looked up
, see section 4.6.3.1 Linux notes.
The RPM files you may want to use are:
MySQL-VERSION.i386.rpm
The MySQL server. You will need this unless you only want to
connect to another MySQL server running on another machine.
MySQL-client-VERSION.i386.rpm
The standard MySQL client programs. You probably always want to
install this package.
MySQL-bench-VERSION.i386.rpm
Tests and benchmarks. Requires Perl and msql-mysql-modules RPMs.
MySQL-devel-VERSION.i386.rpm
Libraries and include files needed if you want to compile other
MySQL clients, such as the Perl modules.
MySQL-VERSION.src.rpm
This contains the source code for all of the above packages. It can also
be used to try to build RPMs for other architectures (for example, Alpha
or SPARC).
To see all files in an RPM package:
shell> rpm -qpl MySQL-VERSION.i386.rpm
To perform a standard minimal installation, run this command:
shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm
To install just the client package:
shell> rpm -i MySQL-client-VERSION.i386.rpm
The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/etc/rc.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously-installed MySQL startup file if you made any changes to it, so you don't lose your changes.)
After installing the RPM file(s), the `mysqld' demon should be running and you should now be able to start using MySQL. See section 4.15 Post-installation setup and testing.
If something goes wrong, can find more information in the binary installation chapter. See section 4.6 Installing a MySQL binary distribution.
If you compile MySQL clients that you've written yourself or that
you obtain from a third party, they must be linked using the
-lmysqlclient
option on the link command. You may also need to
specify a -L
option to tell the linker where to find the library. For
example, if the library is installed in `/usr/local/mysql/lib', use
-L/usr/local/mysql/lib -lmysqlclient
on the link command.
For clients that use MySQL header files, you may need to specify a
-I
option when you compile them (for example,
-I/usr/local/mysql/include
), so the compiler can find the header
files.
The following sections indicate some of the issues that have been observed to occur on particular systems when installing MySQL from a binary distribution.
MySQL needs at least Linux 2.0.
The binary release is linked with -static
, which means you not
normally need not worry about which version of the system libraries you
have. You need not install LinuxThreads, either. A program linked with
-static
is slightly bigger than a dynamically-linked program but
also slightly faster (3-5%). One problem however is that you can't use
user definable functions (UDFs) with a statically-linked program. If
you are going to write or use UDF functions (this is something only for
C or C++ programmers) you must compile MySQL yourself, using
dynamic linking.
If you are using a libc
-based system (instead of a glibc2
system), you will probably get some problems with hostname resolving and
getpwnam() with the binary release. (This is because glibc
unfortunately depends on some external libraries to resolve hostnames
and getwpent() , even when compiled with -static
). In this case
you probably get the following error message when you run
mysql_install_db
:
Sorry, the host 'xxxx' could not be looked up
or the following error when you try to run mysqld with the --user
option:
getpwnam: No such file or directory
You can solve this problem one of the following ways:
tar
distribution) and install this instead.
mysql_install_db --force
; This will not execute the
resolveip
test in mysql_install_db
. The downside is that
you can't use host names in the grant tables; you must use IP numbers
instead (except for localhost
). If you are using an old MySQL
release that doesn't support --force
you have to remove the
resolveip
test in mysql_install
with an editor.
su
instead of using --user
.
The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.
MySQL Perl support requires Perl 5.004_03 or newer.
The binary distribution of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools.
The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL 3.22.26 and newer can also be built with HP's native thread package.
Other configurations that may work:
The following configurations almost definitely won't work:
To install the distribution, use one of the commands below, where
/path/to/depot
is the full pathname of the depot file:
shell> /usr/sbin/swinstall -s /path/to/depot mysql.full
shell> /usr/sbin/swinstall -s /path/to/depot mysql.server
shell> /usr/sbin/swinstall -s /path/to/depot mysql.client
shell> /usr/sbin/swinstall -s /path/to/depot mysql.developer
The depot places binaries and libraries in `/opt/mysql' and data in
`/var/opt/mysql'. The depot also creates the appropriate entries in
`/sbin/init.d' and `/sbin/rc2.d' to start the server automatically
at boot time. Obviously, this entails being root
to install.
To install the HP-UX tar distribution, you must have a copy of GNU tar
.
You need the following tools to build and install MySQL from source:
gunzip
to uncompress the distribution.
tar
to unpack the distribution. GNU tar
is
known to work.
gcc
>= 2.8.1, egcs
>=
1.0.2, SGI C++ and SunPro C++ are some of the compilers that are known to
work. libg++
is not needed when using gcc
. gcc
2.7.x has a bug that makes it impossible to compile some perfectly legal
C++ files, such as `sql/sql_base.cc'. If you only have gcc
2.7.x,
you must upgrade your gcc
to be able to compile MySQL.
make
program. GNU make
is always recommended and is
sometimes required. If you have problems, we recommend trying GNU
make
3.75 or newer.
If you run into problems, PLEASE ALWAYS USE mysqlbug
when
posting questions to mysql@lists.mysql.com. Even if the problem
isn't a bug, mysqlbug
gathers system information that will help others
solve your problem. By not using mysqlbug
, you lessen the likelihood
of getting a solution to your problem! You will find mysqlbug
in the
`scripts' directory after you unpack the distribution. See section 2.3 How to report bugs or problems.
The basic commands you must execute to install a MySQL source
distribution are (from an unpacked tar
file):
shell> configure shell> make shell> make install shell> scripts/mysql_install_db shell> /usr/local/mysql/bin/safe_mysqld &
If you start from a source RPM, then do the following.
shell> rpm --rebuild MySQL-VERSION.src.rpm
This will make a binary RPM that you can install.
You can add new users using the bin/mysql_setpermission
script if
you install the DBI
and Msql-Mysql-modules
Perl modules.
Here follows a more detailed description:
To install a source distribution, follow the steps below, then proceed to section 4.15 Post-installation setup and testing, for post-installation initialization and testing.
tar
archives and have names like `mysql-VERSION.tar.gz', where
VERSION
is a number like 3.23.6-alpha.
shell> gunzip < mysql-VERSION.tar.gz | tar xvf -This command creates a directory named `mysql-VERSION'.
shell> cd mysql-VERSION
shell> ./configure --prefix=/usr/local/mysql shell> makeWhen you run
configure
, you might want to specify some options.
Run ./configure --help
for a list of options.
section 4.7.3 Typical configure
options, discusses some of the
more useful options.
If configure
fails, and you are going to send mail to
lines from `config.log' that you think can help solve the problem. Also
include the last couple of lines of output from configure
if
configure
aborts. Post the bug report using the mysqlbug
script. See section 2.3 How to report bugs or problems.
If the compile fails, see section 4.8 Problems compiling?, for help with
a number of common problems.
shell> make installYou might need to run this command as
root
.
shell> scripts/mysql_install_dbNote that MySQL versions older than 3.22.10 started the MySQL server when you run
mysql_install_db
. This is no
longer true!
DBI
/DBD
interface,
see section 4.10 Perl installation comments.
support-files/mysql.server
to the location where
your system has its startup files. More information can be found in the
support-files/mysql.server
script itself, and in section 4.15.3 Starting and stopping MySQL automatically.
After everything has been installed, you should initialize and test your distribution.
You can start the MySQL server with the following command,
where BINDIR
is the directory in which safe_mysqld
is
installed (`/usr/local/bin' by default):
shell> BINDIR/safe_mysqld &
If that command fails immediately with mysqld daemon ended
then you can
find some information in the file
`mysql-data-directory/'hostname'.err'. The likely reason is that
you already have another mysqld
server running. See section 19.3 Running multiple MySQL servers on the same machine.
See section 4.15 Post-installation setup and testing.
Sometimes patches appear on the mailing list or are placed in the patches area of the MySQL FTP site.
To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree and run these commands:
shell> patch -p1 < patch-file-name shell> rm config.cache shell> make clean
Patches from the FTP site are distributed as plain text files or as files
compressed with gzip
files. Apply a plain patch as shown above for
mailing list patches. To apply a compressed patch, change into the
top-level directory of your MySQL source tree and run these
commands:
shell> gunzip < patch-file-name.gz | patch -p1 shell> rm config.cache shell> make clean
After applying a patch, follow the instructions for a normal source install,
beginning with the ./configure
step. After running the make
install
step, restart your MySQL server.
You may need to bring down any currently running server before you run
make install
. (Use mysqladmin shutdown
to do this.) Some
systems do not allow you to install a new version of a program if it replaces
the version that is currently executing.
configure
options
The configure
script gives you a great deal of control over how you
configure your MySQL distribution. Typically you do this using
options on the configure
command line. You can also affect
configure
using certain environment variables. For a list of options
supported by configure
, run this command:
shell> ./configure --help
Some of the more commonly-used configure
options are described below:
--without-server
option:
shell> ./configure --without-serverIf you don't have a C++ compiler,
mysql
will not compile (it is the
one client program that requires C++). In this case,
you can remove the code in configure
that tests for the C++ compiler
and then run ./configure
with the --without-server
option. The
compile step will still try to build mysql
, but you can ignore any
warnings about `mysql.cc'. (If make
stops, try make -k
to tell it to continue with the rest of the build even if errors occur.)
configure
command something like one
of these:
shell> ./configure --prefix=/usr/local/mysql shell> ./configure --prefix=/usr/local \ --localstatedir=/usr/local/mysql/dataThe first command changes the installation prefix so that everything is installed under `/usr/local/mysql' rather than the default of `/usr/local'. The second command preserves the default installation prefix, but overrides the default location for database directories (normally `/usr/local/var') and changes it to
/usr/local/mysql/data
.
configure
command like this:
shell> ./configure --with-unix-socket-path=/usr/local/mysql/tmp/mysql.sockNote that the given file must be an absolute pathname!
configure
like this:
shell> ./configure --with-client-ldflags=-all-static \ --with-mysqld-ldflags=-all-static
gcc
and don't have libg++
or libstdc++
installed, you can tell configure
to use gcc
as your C++
compiler:
shell> CC=gcc CXX=gcc ./configureWhen you use
gcc
as your C++ compiler, it will not attempt to link in
libg++
or libstdc++
.
If the build fails and produces errors about your compiler or linker not
being able to create the shared library `libmysqlclient.so.#' (`#'
is a version number), you can work around this problem by giving the
--disable-shared
option to configure
. In this case,
configure
will not build a shared libmysqlclient.so.#
library.
DEFAULT
column values for
non-NULL
columns (i.e., columns that are not allowed to be
NULL
). This causes INSERT
statements to generate an error
unless you explicitly specify values for all columns that require a
non-NULL
value. To suppress use of default values, run
configure
like this:
shell> CXXFLAGS=-DDONT_USE_DEFAULT_FIELDS ./configure
--with-charset
option:
shell> ./configure --with-charset=CHARSET
CHARSET
may be one of big5
, cp1251
, cp1257
,
czech
, danish
,dec8
, dos
, euc_kr
,
gb2312
gbk
, german1
, hebrew
, hp8
,
hungarian
, koi8_ru
, koi8_ukr
, latin1
, latin2
,
sjis
, swe7
, tis620
, ujis
, usa7
,
win1251
or win1251ukr
.
See section 9.1.1 The character set used for data and sorting.
Note that if you want to change the character set, you must do a make
distclean
between configurations!
If you want to convert characters between the server and the client,
you should take a look at the SET OPTION CHARACTER SET
command.
See section 7.25 SET OPTION
syntax.
Warning: If you change character sets after having created any
tables, you will have to run myisamchk -r -q
on every table. Your
indexes may be sorted incorrectly otherwise. (This can happen if you
install MySQL, create some tables, then reconfigure
MySQL to use a different character set and reinstall it.)
--with-debug
option:
shell> ./configure --with-debugThis causes a safe memory allocator to be included that can find some errors and that provides output about what is happening. See section G.1 Debugging a MySQL server.
All MySQL programs compile cleanly for us with no warnings on
Solaris using gcc
. On other systems, warnings may occur due to
differences in system include files. See section 4.9 MIT-pthreads notes, for warnings
that may occur when using MIT-pthreads. For other problems, check the list
below.
The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:
configure
is run after it already has been run, it may use
information that was gathered during its previous invocation. This
information is stored in `config.cache'. When configure
starts
up, it looks for that file and reads its contents if it exists, on the
assumption that the information is still correct. That assumption is invalid
when you reconfigure.
configure
, you must run make
again
to recompile. However, you may want to remove old object files from previous
builds first, since they were compiled using different configuration options.
To prevent old configuration information or object files from being used,
run these commands before rerunning configure
:
shell> rm config.cache shell> make clean
Alternatively, you can run make distclean
.
The list below describes some of the problems compiling MySQL that have been found to occur most often:
Internal compiler error: program cc1plus got fatal signal 11 or Out of virtual memory or Virtual memory exhaustedThe problem is that
gcc
requires huge amounts of memory to compile
`sql_yacc.cc' with inline functions. Try running configure
with
the --with-low-memory
option:
shell> ./configure --with-low-memoryThis option causes
-fno-inline
to be added to the compile line if you
are using gcc
and -O0
if you are using something else. You
should try the --with-low-memory
option even if you have so much
memory and swap space that you think you can't possibly have run out. This
problem has been observed to occur even on systems with generous hardware
configurations, and the --with-low-memory
option usually fixes it.
configure
picks c++
as the compiler name and
GNU c++
links with -lg++
. If you are using gcc
,
that behavior can cause problems during configuration such as this:
configure: error: installation or configuration problem: C++ compiler cannot create executables.You might also observe problems during compilation related to
g++
, libg++
or libstdc++
.
One cause of these problems is that you may not have g++
, or you may
have g++
but not libg++
or libstdc++
. Take a look at
the `config.log' file. It should contain the exact reason why your c++
compiler didn't work! To work around these problems, you can use gcc
as your C++ compiler. Try setting the environment variable CXX
to
"gcc -O3"
. For example:
shell> CXX="gcc -O3" ./configureThis works because
gcc
compiles C++ sources as well as g++
does, but does not link in libg++
or libstdc++
by default.
Another way to fix these problems, of course, is to install g++
,
libg++
and libstdc++
.
make
to GNU make
:
making all in mit-pthreads make: Fatal error in reader: Makefile, line 18: Badly formed macro assignment or make: file `Makefile' line 18: Must be a separator (: or pthread.h: No such file or directorySolaris and FreeBSD are known to have troublesome
make
programs.
GNU make
version 3.75 is known to work.
CFLAGS
and CXXFLAGS
environment
variables. You can also specify the compiler names this way using CC
and CXX
. For example:
shell> CC=gcc shell> CFLAGS=-O6 shell> CXX=gcc shell> CXXFLAGS=-O6 shell> export CC CFLAGS CXX CXXFLAGSSee section 4.14 TcX binaries, for a list of flag definitions that have been found to be useful on various systems.
gcc
compiler:
client/libmysql.c:273: parse error before `__attribute__'
gcc
2.8.1 is known to work, but we recommend using egcs
1.0.3a or newer instead.
mysqld
,
configure
didn't correctly detect the type of the last argument to
accept()
, getsockname()
or getpeername()
:
cxx: Error: mysqld.cc, line 645: In this statement, the referenced type of the pointer value "&length" is "unsigned long", which is not compatible with "int". new_sock = accept(sock, (struct sockaddr *)&cAddr, &length);To fix this, edit the `config.h' file (which is generated by
configure
). Look for these lines:
/* Define as the base type of the last arg to accept */ #define SOCKET_SIZE_TYPE XXXChange
XXX
to size_t
or int
, depending on your
operating system. (Note that you will have to do this each time you run
configure
, since configure
regenerates `config.h'.)
"sql_yacc.yy", line xxx fatal: default action causes potential...This is a sign that your version of
yacc
is deficient.
You probably need to install bison
(the GNU version of yacc
)
and use that instead.
mysqld
or a MySQL client, run
configure
with the --with-debug
option, then recompile and
link your clients with the new client library.
See section G.2 Debugging a MySQL client.
This section describes some of the issues involved in using MIT-pthreads.
Note that on Linux you should NOT use MIT-pthreads but install LinuxThreads! See section 4.11.5 Linux notes (all Linux versions).
If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes most FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 4.2 Operating systems supported by MySQL.
configure
with the --with-mit-threads
option:
shell> ./configure --with-mit-threadsBuilding in a non-source directory is not supported when using MIT-pthreads, because we want to minimize our changes to this code.
AF_UNIX
protocol used to implement
Unix sockets. This means that if you compile using MIT-pthreads, all
connections must be made using TCP/IP (which is a little slower). If you
find after building MySQL that you cannot connect to the local
server, it may be that your client is attempting to connect to
localhost
using a Unix socket as the default. Try making a TCP/IP
connection with mysql
by using a host option (-h
or
--host
) to specify the local host name explicitly.
--without-server
to build only the client code, clients will not know whether or not
MIT-pthreads is being used and will use Unix socket connections by default.
Since Unix sockets do not work under MIT-pthreads, this means you will need
to use -h
or --host
when you run client programs.
--use-locking
option.
bind()
command fails to bind to a socket without
any error message (at least on Solaris). The result is that all connections
to the server fail. For example:
shell> mysqladmin version mysqladmin: connect to server at '' failed; error: 'Can't connect to mysql server on localhost (146)'The solution to this is to kill the
mysqld
server and restart it.
This has only happened to us when we have forced the server down and done
a restart immediately.
sleep()
system call isn't interruptible with
SIGINT
(break). This is only noticeable when you run mysqladmin
--sleep
. You must wait for the sleep()
call to terminate before the
interrupt is served and the process stops.
ld: warning: symbol `_iob' has differing sizes: (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4; file /usr/lib/libc.so value=0x140); /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken ld: warning: symbol `__iob' has differing sizes: (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4; file /usr/lib/libc.so value=0x140); /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken
implicit declaration of function `int strtoll(...)' implicit declaration of function `int strtoul(...)'
readline
to work with MIT-pthreads. (This isn't
needed, but may be interesting for someone.)
Perl support for MySQL is provided by means of the
DBI
/DBD
client interface. See section 20.5 MySQL Perl API. The Perl
DBD
/DBI
client code requires Perl 5.004 or later. The
interface will not work if you have an older version of Perl.
MySQL Perl support also requires that you've installed MySQL client programming support. If you installed MySQL from RPM files, client programs are in the client RPM, but client programming support is in the developer RPM. Make sure you've installed the latter RPM.
As of release 3.22.8, Perl support is distributed separately from the main MySQL distribution. If you want to install Perl support, the files you will need can be obtained from http://www.mysql.com/Contrib.
The Perl distributions are provided as compressed tar
archives and
have names like `MODULE-VERSION.tar.gz', where MODULE
is the
module name and VERSION
is the version number. You should get the
Data-Dumper
, DBI
, and Msql-Mysql-modules
distributions
and install them in that order. The installation procedure is shown below.
The example shown is for the Data-Dumper
module, but the procedure is
the same for all three distributions.
shell> gunzip < Data-Dumper-VERSION.tar.gz | tar xvf -This command creates a directory named `Data-Dumper-VERSION'.
shell> cd Data-Dumper-VERSION
shell> perl Makefile.PL shell> make shell> make test shell> make install
The make test
command is important, because it verifies that the
module is working. Note that when you run that command during the
Msql-Mysql-modules
installation to exercise the interface code, the
MySQL server must be running or the test will fail.
It is a good idea to rebuild and reinstall the Msql-Mysql-modules
distribution whenever you install a new release of MySQL,
particularly if you notice symptoms such as all your DBI
scripts
dumping core after you upgrade MySQL.
If you don't have the right to install Perl modules in the system directory or if you to install local Perl modules, the following reference may help you:
http://www.iserver.com/support/contrib/perl5/modules.html
Look under the heading
Installing New Modules that Require Locally Installed Modules
.
To install the MySQL DBD
module with ActiveState Perl on
Win32, you should do the following:
set HTTP_proxy=my.proxy.com:3128
C:\perl\bin\ppm.pl
DBI
: install DBI
DBD::mysql:
http://www.mysql.com/Contrib/ppd/DBD-mysql.ppd
If you can't get the above to work, you should instead install the MyODBC driver and connect to MySQL server through ODBC.
use DBI; $dbh= DBI->connect("DBI:ODBC:$dsn","$user","$password") || die "Got error $DBI::errstr when connecting to $dsn\n";
The MySQL Perl distribution contains DBI
,
DBD:MySQL
and DBD:ODBC
.
C:
so that you get a `C:\PERL' directory.
perl
works by executing perl -v
in a DOS shell.
DBI
/DBD
interface
If Perl reports that it can't find the ../mysql/mysql.so
module,
then the problem is probably that Perl can't locate the shared library
`libmysqlclient.so'.
You can fix this by any of the following methods:
Msql-Mysql-modules
distribution with perl
Makefile.PL -static
rather than perl Makefile.PL
libmysqlclient.so
to the directory where your other shared
libraries are located (probably `/usr/lib' or `/lib').
Linux
you can add the pathname of the directory where
libmysqlclient.so
is located to the `/etc/ld.so.conf' file.
libmysqlclient.so
is located
to the LD_RUN_PATH
environment variable.
If you get the following errors from DBD-mysql
,
you are probably using gcc
(or using an old binary compiled with
gcc
):
/usr/bin/perl: can't resolve symbol '__moddi3' /usr/bin/perl: can't resolve symbol '__divdi3'
Add -L/usr/lib/gcc-lib/... -lgcc
to the link command when the
`mysql.so' library gets built (check the output from make
for
`mysql.so' when you compile the Perl client). The -L
option
should specify the pathname of the directory where `libgcc.a' is located
on your system.
Another cause of this problem may be that Perl and MySQL aren't both
compiled with gcc
. In this case, you can solve the mismatch by
compiling both with gcc
.
If you want to use the Perl module on a system that doesn't support dynamic
linking (like SCO) you can generate a static version of Perl that includes
DBI
and DBD-mysql
. The way this works is that you generate a
version of Perl with the DBI
code linked in and install it on top of
your current Perl. Then you use that to build a version of Perl that
additionally has the DBD
code linked in, and install that.
On SCO, you must have the following environment variables set:
shell> LD_LIBRARY_PATH=/lib:/usr/lib:/usr/local/lib:/usr/progressive/lib or shell> LD_LIBRARY_PATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib shell> LIBPATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib shell> MANPATH=scohelp:/usr/man:/usr/local1/man:/usr/local/man:/usr/skunk/man:
First, create a Perl that includes a statically-linked DBI
by running
these commands in the directory where your DBI
distribution is
located:
shell> perl Makefile.PL LINKTYPE=static shell> make shell> make install shell> make perl
Then you must install the new Perl. The output of make perl
will
indicate the exact make
command you will need to execute to perform
the installation. On SCO, this is make -f Makefile.aperl inst_perl
MAP_TARGET=perl
.
Next, use the just-created Perl to create another Perl that also includes a
statically-linked DBD::mysql
by running these commands in the
directory where your Msql-Mysql-modules
distribution is located:
shell> perl Makefile.PL LINKTYPE=static shell> make shell> make install shell> make perl
Finally, you should install this new Perl. Again, the output of make
perl
indicates the command to use.
The following sections indicate some of the issues that have been observed to occur on particular systems when installing MySQL from a source distribution.
On Solaris, you may run into trouble even before you get the MySQL
distribution unpacked! Solaris tar
can't handle long file names, so
you may see an error like this when you unpack MySQL:
x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks tar: directory checksum error
In this case, you must use GNU tar
(gtar
) to unpack the
distribution. You can find a precompiled copy for Solaris at
http://www.mysql.com/Downloads/.
Sun native threads work only on Solaris 2.5 and higher. For 2.4 and earlier versions, MySQL will automatically use MIT-pthreads. See section 4.9 MIT-pthreads notes.
If you get the following error from configure:
checking for restartable system calls... configure: error can not run test programs while cross compiling
This means that you have something wrong with your compiler installation!
In this case you should upgrade your compiler to a newer version. You may
also be able to solve this problem by inserting the following row into the
config.cache
file:
ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}
If you are using Solaris on a SPARC, the recommended compiler is egcs
1.1.2 or newer. You can find this at http://egcs.cygnus.com/.
Note that egs
1.1.1 and gcc
2.8.1 don't work reliably on SPARC!
The recommended configure
line when using egcs
1.1.2 is:
shell> CC=gcc CFLAGS="-O6" \ CXX=gcc CXXFLAGS="-O6 -felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql --with-low-memory
If you have the Sun Workshop 4.2 compiler, you can run configure
like
this:
CC=cc CFLAGS="-xstrconst -Xa -xO4 -native -mt" CXX=CC CXXFLAGS="-xO4 -native -noex -mt" ./configure --prefix=/usr/local/mysql
shell> CC=cc CFLAGS="-Xa -fast -xO4 -native -xstrconst -mt" \ CXX=CC CXXFLAGS="-noex -XO4 -mt" \ ./configure
You may also have to edit the configure
script to change this line:
#if !defined(__STDC__) || __STDC__ != 1
to this:
#if !defined(__STDC__)
If you turn on __STDC__
with the -Xc
option, the Sun compiler
can't compile with the Solaris `pthread.h' header file. This is a Sun
bug (broken compiler or broken include file).
If mysqld
issues the error message shown below when you run it, you have
tried to compile MySQL with the Sun compiler without enabling the
multi-thread option (-mt
):
libc internal error: _rmutex_unlock: rmutex not held
Add -mt
to CFLAGS
and CXXFLAGS
and try again.
If you get the following error when compiling MySQL with gcc
,
it means that your gcc
is not configured for your version of Solaris!
shell> gcc -O3 -g -O2 -DDBUG_OFF -o thr_alarm ... ./thr_alarm.c: In function `signal_hand': ./thr_alarm.c:556: too many arguments to function `sigwait'
The proper thing to do in this case is to get the newest version of
egcs
and compile it with your current gcc
compiler! At
least for Solaris 2.5, almost all binary versions of gcc
have
old, unusable include files that will break all programs that use
threads (and possibly other programs)!
Solaris doesn't provide static versions of all system libraries
(libpthreads
and libdl
), so you can't compile MySQL
with --static
. If you try to do so, you will get the error:
ld: fatal: library -ldl: not found
If too many processes try to connect very rapidly to mysqld
, you will
see this error in the MySQL log:
Error in accept: Protocol error
You might try starting the server with the --set-variable back_log=50
option as a workaround for this.
If you are linking your own MySQL client, you might get the following error when you try to execute it:
ld.so.1: ./my: fatal: libmysqlclient.so.#: open failed: No such file or directory
The problem can be avoided by one of the following methods:
-Lpath
):
-Wl,r/full-path-to-libmysqlclient.so
.
libmysqclient.so
to `/usr/lib'.
libmysqlclient.so
is located
to the LD_RUN_PATH
environment variable before running your client.
You can normally use a Solaris 2.6 binary on Solaris 2.7. Most of the Solaris 2.6 issues also apply for Solaris 2.7.
Note that MySQL 3.23.4 and above should be able to autodetect Solaris 2.7 and enable workarounds for the following problems!
Solaris 2.7 has some bugs in the include files. You may see the following
error when you use gcc
:
/usr/include/widec.h:42: warning: `getwc' redefined /usr/include/wchar.h:326: warning: this is the location of the previous definition
If this occurs, you can do the following to fix the problem:
Copy /usr/include/widec.h
to
.../lib/gcc-lib/os/gcc-version/include
and change line 41 from:
#if !defined(lint) && !defined(__lint) to #if !defined(lint) && !defined(__lint) && !defined(getwc)
Alternatively, you can edit `/usr/include/widec.h' directly. Either
way, after you make the fix, you should remove `config.cache' and run
configure
again!
If you get errors like this when you run make
, it's because configure
didn't detect the `curses.h' file (probably because of the error in
/usr/include/widec.h
:
In file included from mysql.cc:50: /usr/include/term.h:1060: syntax error before `,' /usr/include/term.h:1081: syntax error before `;'
The solution to this is to do one of the following steps:
#define HAVE_TERM
line from `config.h' file and
run make
again.
CFLAGS=-DHAVE_CURSES CXXFLAGS=-DHAVE_CURSES ./configure
If you are using gcc
or egcs
on Solaris x86 and you
experience problems with core dumps under load, you should use the
following configure
command:
shell> CC=gcc CFLAGS="-O6 -fomit-frame-pointer" \ CXX=gcc \ CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql
This will avoid problems with the libstdc++
library and with C++
exceptions.
If this doesn't help, you should compile a debug version and run
it with a trace file or under gdb
. See section G.1 Debugging a MySQL server.
On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn
means you will need GNU make
.
Some SunOS 4 systems have problems with dynamic libraries and
libtool
. You can use the following configure
line to avoid
this problem:
shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static
When compiling readline
, you may get warnings about duplicate defines.
These may be ignored.
When compiling mysqld
, there will be some implicit declaration
of function
warnings. These may be ignored.
MySQL uses LinuxThreads on Linux. If you are using an old Linux
version that doesn't have glibc2
, you must install LinuxThreads before
trying to compile
MySQL. http://www.mysql.com/Downloads/Linux
If you can't start mysqld
or if mysql_install_db
doesn't work,
please continue reading! This only happens on Linux system with problems in
the LinuxThreads or libc
/glibc
libraries. There are a lot of
simple workarounds to get MySQL to work! The simplest is to use the
binary version of MySQL (not the RPM) for Linux x86. One nice
aspect of this version is that it's probably 10% faster than any version you
would compile yourself! See section 10.2.1 How compiling and linking affects the speed of MySQL.
One known problem with the binary distribution is that with older Linux
systems that use libc
(like RedHat 4.x or Slackware), you will get
some non-fatal problems with hostname resolution
See section 4.6.3.1 Linux notes.
myisamchk
hangs with libc.so.5.3.12
. Upgrading to the newest
libc
fixes this problem.
When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.
If you see a dead mysqld
daemon process with ps
, this usually
means that you have found a bug in MySQL or you have got a corrupted
table. See section 18.1 What to do if MySQL keeps crashing.
If you are using LinuxThreads and mysqladmin shutdown
doesn't work,
you must upgrade to LinuxThreads 0.7.1 or newer.
If you are using RedHat, you might get errors like this:
/usr/bin/perl is needed... /usr/sh is needed... /usr/sh is needed...
If so, you should upgrade your version of rpm
to
`rpm-2.4.11-1.i386.rpm' and `rpm-devel-2.4.11-1.i386.rpm' (or later).
You can get the upgrades of libraries to RedHat 4.2 from ftp://ftp.redhat.com/updates/4.2/i386. Or http://www.sunsite.unc.edu/pub/Linux/distributions/redhat/code/rpm/ for other distributions.
If you are linking your own MySQL client and get the error:
ld.so.1: ./my: fatal: libmysqlclient.so.4: open failed: No such file or directory
when executing them, the problem can be avoided by one of the following methods:
-Lpath
):
-Wl,r/path-libmysqlclient.so
.
libmysqclient.so
to `/usr/lib'.
libmysqlclient.so
is located
to the LD_RUN_PATH
environment variable before running your client.
If you are using the Fujitsu compiler (fcc / FCC)
you will have
some problems compiling MySQL because the Linux header files are very
gcc
oriented.
The following configure
line should work with fcc/FCC
:
CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-low-memory
MySQL requires libc
version 5.4.12 or newer. It's known to
work with libc
5.4.46. glibc
version 2.0.6 and later should
also work. There have been some problems with the glibc
RPMs from
RedHat so if you have problems, check whether or not there are any updates!
The glibc
2.0.7-19 and 2.0.7-29 RPMs are known to work.
On some older Linux distributions, configure
may produce an error
like this:
Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file. See the Installation chapter in the Reference Manual.
Just do what the error message says and add an extra underscore to the
_P
macro that has only one underscore, then try again.
You may get some warnings when compiling; those shown below can be ignored:
mysqld.cc -o objs-thread/mysqld.o mysqld.cc: In function `void init_signals()': mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int' mysqld.cc: In function `void * signal_hand(void *)': mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int'
In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following:
shell> cp support-files/mysql.server /etc/init.d/mysql.server shell> /usr/sbin/update-rc.d mysql.server defaults 99
mysql.server
can be found in the `share/mysql' directory
under the MySQL installation directory, or in the
`support-files' directory of the MySQL source tree.
If mysqld
always core dumps when it starts up, the problem may be that
you have an old `/lib/libc.a'. Try renaming it, then remove
`sql/mysqld' and do a new make install
and try again. This
problem has been reported on some Slackware installations. RedHat 5.0 has
also a similar problem with some new glibc
versions.
See section 4.11.5.2 RedHat 5.0 notes.
If you get the following error when linking mysqld
,
it means that your `libg++.a' is not installed correctly:
/usr/lib/libc.a(putc.o): In function `_IO_putc': putc.o(.text+0x0): multiple definition of `_IO_putc'
You can avoid using `libg++.a' by running configure
like this:
shell> CXX=gcc ./configure
If you have any problems with MySQL on RedHat, you should start by
upgrading glibc
to the newest possible version!
If you install all the official RedHat patches (including
glibc-2.0.7-19
and glibc-devel-2.0.7-19
), both the
binary and source distributions of MySQL should work without
any trouble!
The updates are needed since there is a bug in glibc
2.0.5 in how
pthread_key_create
variables are freed. With glibc
2.0.5, you
must use a statically-linked MySQL binary distribution. If you
want to compile from source, you must install the corrected version of
LinuxThreads from http://www.mysql.com/Downloads/Linux or upgrade your
glibc
.
If you have an incorrect version of glibc
or LinuxThreads, the symptom
is that mysqld
crashes after each connection. For example,
mysqladmin version
will crash mysqld
when it finishes!
Another symptom of incorrect libraries is that mysqld
crashes at
once when it starts. On some Linux systems, this can be fixed by configuring
like this:
shell> ./configure --with-mysqld-ldflags=-all-static
On Redhat 5.0, the easy way out is to install the glibc
2.0.7-19 RPM
and run configure
without the
--with-mysqld-ldflags=-all-static
option.
For the source distribution of glibc
2.0.7, a patch that is easy to
apply and is tested with MySQL may be found at:
http://www.mysql.com/Download/Linux/glibc-2.0.7-total-patch.tar.gz
If you experience crashes like these when you build MySQL, you can always download the newest binary version of MySQL. This is statically-linked to avoid library conflicts and should work on all Linux systems!
MySQL comes with an internal debugger that can generate trace files with a lot of information that can be used to find and solve a wide range of different problems. See section G.1 Debugging a MySQL server.
The glibc
of RedHat 5.1 (glibc
2.0.7-13) has a memory leak, so
to get a stable MySQL version, you must upgrade glibc
to
2.0.7-19, downgrade glibc
or use a binary version of mysqld
. If
you don't do this, you will encounter memory problems (out of memory, etc.,
etc.). The most common error in this case is:
Can't create a new thread (errno 11). If you are not out of available memory, you can consult the manual for any possible OS dependent bug
After you have upgraded to glibc
2.0.7-19, you can configure
MySQL with dynamic linking (the default), but you cannot
run configure
with the --with-mysqld-ldflags=-all-static
option
until you have installed glibc
2.0.7-19 from source!
You can check which version of glibc
you have with rpm -q glibc
.
In some implementations, readdir_r()
is broken. The symptom is that
SHOW DATABASES
always returns an empty set. This
can be fixed by removing HAVE_READDIR_R
from `config.h' after
configuring and before compiling.
Some problems will require patching your Linux installation. The patch can
be found at
http://www.mysql.com/patches/Linux-sparc-2.0.30.diff. This patch is
against the Linux distribution `sparclinux-2.0.30.tar.gz' that is
available at vger.rutgers.edu
(a version of Linux that was
never merged with the official 2.0.30). You must also install
LinuxThreads 0.6 or newer.
Thanks to jacques@solucorp.qc.ca for this information.
The big problem on Linux-Alpha is that there are still some problems with
threads in glibc
on this platform. You should start by getting the
newest glibc
version you can find.
Note that before you run any programs that use threads (like mysqld
,
thr_alarm
or thr_lock
), you should raise the shared memory
limit (with ulimit
). The MySQL benchmarks are known to fail
if you forget to do this!
Configure MySQL with the following command:
shell> CC=gcc CCFLAGS="-Dalpha_linux_port" \ CXX=gcc CXXFLAGS="-O3 -Dalpha_linux_port -felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql
Try to compile mysys/thr_lock
and mysys/thr_alarm
.
Test that these programs work! (Invoke each one with no arguments.
Each should end with test_succeeded
if everything
was okay.)
After installing MySQL, uncomment the ulimit
command in
safe_mysqld
and add options to increase shared memory.
Note that Linux-Alpha is still an alpha-quality platform for MySQL.
With the newest glibc
, you have a very good chance of it working.
If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:
shell> CFLAGS=-DDONT_USE_THR_ALARM \ CXXFLAGS=-DDONT_USE_THR_ALARM \ ./configure ...
This doesn't affect the performance of MySQL, but has the side
effect that you can't kill clients that are ``sleeping'' on a connection with
mysqladmin kill
or mysqladmin shutdown
. Instead, the client
will die when it issues its next command.
MySQL should work on MkLinux with the newest glibc
package
(tested with glibc
2.0.7).
To get MySQL to work on Qube2, (Linux Mips), you need the newest
glibc
libraries (glibc-2.0.7-29C2
is known to work). You must also use
the egcs
C++ compiler (egcs-1.0.2-9
or newer).
When compiling threaded programs under Digital UNIX, the documentation
recommends using the -pthread
option for cc
and cxx
and
the libraries -lmach -lexc
(in addition to -lpthread
). You
should run configure
something like this:
shell> CC="cc -pthread" CXX="cxx -pthread -O" \ ./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"
When compiling mysqld
, you may see a couple of warnings like this:
mysqld.cc: In function void handle_connections()': mysqld.cc:626: passing long unsigned int *' as argument 3 of accept(int,sockadddr *, int *)'
You can safely ignore these warnings. They occur because configure
can detect only errors, not warnings.
If you start the server directly from the command line, you may have problems
with it dying when you log out. (When you log out, your outstanding processes
receive a SIGHUP
signal.) If so, try starting the server like this:
shell> nohup mysqld [options] &
nohup
causes the command following it to ignore any SIGHUP
signal sent from the terminal. Alternatively, start the server by running
safe_mysqld
, which invokes mysqld
using nohup
for you.
If you have problems compiling and have DEC CC
and gcc
installed, try running configure
like this:
shell> CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql
If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with:
shell> touch include/c_asm.h shell> CC=gcc CFLAGS=-I./include \ CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql
On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital UNIX V4.0 (Rev. 878)"
the compiler had some strange behavior (undefined asm
symbols).
/bin/ld
also appears to be broken (problems with _exit
undefined
errors occuring while linking mysqld
). On this system, we
have managed to compile MySQL with the following configure
line, after replacing /bin/ld
with the version from OSF 4.0C:
shell> CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
In some versions of OSF1, the alloca()
function is broken. Fix
this by removing the line in `config.h' that defines 'HAVE_ALLOCA'
.
The alloca()
function also may have an incorrect prototype in
/usr/include/alloca.h
. This warning resulting from this can be ignored.
configure
will use the following thread libraries automatically:
--with-named-thread-libs="-lpthread -lmach -lexc -lc"
.
When using gcc
, you can also try running configure
like this:
shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ....
If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:
shell> CFLAGS=-DDONT_USE_THR_ALARM \ CXXFLAGS=-DDONT_USE_THR_ALARM \ ./configure ...
This doesn't affect the performance of MySQL, but has the side
effect that you can't kill clients that are ``sleeping'' on a connection with
mysqladmin kill
or mysqladmin shutdown
. Instead, the client
will die when it issues its next command.
You may have to undefine some things in `config.h' after running
configure
and before compiling.
In some Irix implementations, the alloca()
function is broken. If the
mysqld
server dies on some SELECT
statements, remove the lines
from `config.h' that define HAVE_ALLOC
and HAVE_ALLOCA_H
.
If mysqladmin create
doesn't work, remove the line from
`config.h' that defines HAVE_READDIR_R
. You may have to remove
the HAVE_TERM_H
line as well.
SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html
At the very minimum, you should install the latest kernel rollup, the
latest rld
rollup, and the latest libc
rollup.
You definately need all the POSIX patches on this page, for pthreads support:
http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html
If you get the something like the following error when compiling `mysql.cc':
"/usr/include/curses.h", line 82: error(1084): invalid combination of type
Then type the following in the top-level directory of your MySQL source tree:
shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h shell> make
There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly-understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.
If you are compiling with gcc
, you can use the following
configure
command:
shell> CC=gcc CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql --with-thread-safe-client --with-named-thread-libs=-lpthread
FreeBSD 3.x is recommended for running MySQL since it the thread package is much more integrated.
The easiest and therefor the preferred way to install is to use the mysql-server and mysql-client ports available on http://www.freebsd.org
Using these gives you:
It is recomended to use MIT-pthreads on FreeBSD 2.x and native threads on versions 3 and up. It is possible to run with with native threads on some late 2.2.x versions but you may encounter problems shutting down mysqld.
Be sure to have your name resolver setup correct. Otherwise you may experience resolver delays or failures when connecting to mysqld.
Make sure that the localhost
entry in the `/etc/hosts' file is
correct (otherwise you will have problems connecting to the database). The
`/etc/hosts' file should start with a line:
127.0.0.1 localhost localhost.your.domain
If you notice that configure
will use MIT-pthreads, you should read
the MIT-pthreads notes. See section 4.9 MIT-pthreads notes.
If you get an error from make install
that it can't find
`/usr/include/pthreads', configure
didn't detect that you need
MIT-pthreads. This is fixed by executing these commands:
shell> rm config.cache shell> ./configure --with-mit-threads
The behavior of FreeBSD make
is slightly different from that of GNU
make
. If you have make
-related problems, you should install GNU
make
.
FreeBSD is also known to have a very low default file handle limit. See section 18.11 File not found. Uncomment the ulimit -n section in safe_mysqld or raise the limits for the mysqld user in /etc/login.conf (and rebuild it witg cap_mkdb /etc/login.conf) also be sure you set the appropriate Class for this user in the password file if you are not using the default (use: chpass mysqld-user-name)
If you have a problem with SELECT NOW()
returning values in GMT and
not your local time, you have to set the TZ
environment variable to
your current timezone. This should be done for the environment in which
the server runs, for example, in safe_mysqld
or mysql.server
.
To get a secure and stable system you should only use FreeBSD kernels
that are marked -STABLE
To compile on NetBSD you need GNU make
. Otherwise the compile will crash
when make
tries to run lint
on C++ files.
If you get the following error when compiling MySQL, your
ulimit
value for virtual memory is too low:
item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)': item_func.h:28: virtual memory exhausted make[2]: *** [item_func.o] Error 1
Try using ulimit -v 80000
and run make
again. If this
doesn't work and you are using bash
, try switching to csh
or sh
; some BSDI users have reported problems with bash
and ulimit
.
If you are using gcc
, you may also use have to use the
--with-low-memory
flag for configure
to be able to compile
`sql_yacc.cc'.
If you have a problem with SELECT NOW()
returning values in GMT and
not your local time, you have to set the TZ
environment variable to
your current timezone. This should be done for the environment in which
the server runs, for example in safe_mysqld
or mysql.server
.
Upgrade to BSD/OS 3.1. If that is not possible, install BSDIpatch M300-038.
Use the following command when configuring MySQL:
shell> env CXX=shlicc++ CC=shlicc2 \ ./configure \ --prefix=/usr/local/mysql \ --localstatedir=/var/mysql \ --without-perl \ --with-unix-socket-path=/var/mysql/mysql.sock
The following is also known to work:
shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \ ./configure \ --prefix=/usr/local/mysql \ --with-unix-socket-path=/var/mysql/mysql.sock
You can change the directory locations if you wish, or just use the defaults by not specifying any locations.
If you have problems with performance under heavy load, try using the
--skip-thread-priority
option to safe_mysqld
! This will run
all threads with the same priority; on BSDI 3.1, this gives better
performance (at least until BSDI fixes their thread scheduler).
If you get the error virtual memory exhausted
while compiling,
you should try using ulimit -v 80000
and run make
again.
If this doesn't work and you are using bash
, try switching to
csh
or sh
; some BSDI users have reported problems with
bash
and ulimit
.
BSDI 4.x has some thread related bugs. If you want to use MySQL on this, you should install all thread related patches. At least M400-023 should be installed.
The current port is tested only on a ``sco3.2v5.0.4'' and ``sco3.2v5.0.5'' system. There has also been a lot of progress on a port to ``sco 3.2v4.2''.
For the moment the recommended compiler on OpenServer is gcc 2.95.2. With this
you should be able to compile MySQL
with just:
CC=gcc CXX=gcc ./configure ... (options)
gcc
2.7.2 in Skunkware 97 does not have
GNU as
. You can also use egcs
1.1.2 or newer
http://www.egcs.com/. If you are using egcs
1.1.2 you have
to execute the following command:
shell> cp -p /usr/include/pthread/stdtypes.h /usr/local/lib/gcc-lib/i386-pc-sco3.2v5.0.5/egcs-2.91.66/include/pthread/
./configure
in the `threads/src' directory and select
the SCO OpenServer option. This command copies `Makefile.SCO5' to
`Makefile'.
make
.
cd
to the `thread/src' directory, and run make
install
.
make
when making MySQL.
shell> CC="gcc -DSCO" CXX="gcc -DSCO" ./configureThe
-DSCO
is needed to help configure detect some thread
functions properly. If you forget -DSCO
, you will get the following
error message while compiling:
my_pthread.c: In function `my_pthread_mutex_init': my_pthread.c:374: `pthread_mutexattr_default' undeclared (first use this function)
safe_mysqld
as root, you probably will get only the
default 110 open files per process. mysqld
will write a note about this
in the log file.
configure
command should work:
shell> CC="gcc -belf" ./configure --prefix=/usr/local/mysql --disable-shared
configure
command should work:
shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \ ./configure \ --with-debug --prefix=/usr/local/mysql \ --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \ --with-named-curses-libs="-lcurses"You may get some problems with some include files. In this case, you can find new SCO-specific include files at ftp://www.mysql.com/pub/mysql/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the `include' directory of your MySQL source tree.
SCO development notes:
mysqld
with -lgthreads -lsocket -lgthreads
.
www.mysql.com
) comes linked with
GNU malloc
. If you encounter problems with memory usage, make sure that
`gmalloc.o'
is included in `libgthreads.a' and `libgthreads.so'.
read()
,
write()
, getmsg()
, connect()
, accept()
,
select()
and wait()
.
If you want to install DBI on SCO, you have to edit the `Makefiles' in DBI-xxx and each subdirectory:
OLD: NEW: CC = cc CC = gcc -belf CCCDLFLAGS = -KPIC -W1,-Bexport CCCDLFLAGS = -fpic CCDLFLAGS = -wl,-Bexport CCDLFLAGS = LD = ld LD = gcc -belf -G -fpic LDDLFLAGS = -G -L/usr/local/lib LDDLFLAGS = -L/usr/local/lib LDFLAGS = -belf -L/usr/local/lib LDFLAGS = -L/usr/local/lib LD = ld LD = gcc -belf -G -fpic OPTIMISE = -Od OPTIMISE = -O1 OLD: CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include NEW: CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include
This is because the Perl dynaloader will not load the DBI
modules
if they were compiled with icc
or cc
.
Perl works best when compiled with cc
.
You must use a version of MySQL at least as recent as 3.22.13, since that version fixes some portability problems under Unixware.
We have been able to compile MySQL with the following configure
command on UnixWare 7.0.1:
shell> CC=cc CXX=CC ./configure --prefix=/usr/local/mysql
Automatic detection of xlC
is missing from Autoconf, so a
configure
command something like this is needed when using the IBM
compiler:
shell> CC="xlc_r -ma -O3 -qstrict -DHAVE_INT_8_16_32" \ CXX="xlC_r -ma -O3 -qstrict -DHAVE_INT_8_16_32" \ ./configure
If you are using egcs
to compile MySQL, you
MUST use the -fno-exceptions
flag, as the exception
handling in egcs
is not thread-safe! (This is tested with
egcs
1.1.) We recommend the following configure
line with
egcs
and gcc
on AIX:
shell> CXX=gcc \ CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/home/monty --with-debug --with-low-memory
If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:
shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \ CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \ ./configure --prefix=/home/monty --with-debug --with-low-memory
This doesn't affect the performance of MySQL, but has the side
effect that you can't kill clients that are ``sleeping'' on a connection with
mysqladmin kill
or mysqladmin shutdown
. Instead, the client
will die when it issues its next command.
There are a couple of ``small'' problems when compiling MySQL on
HP-UX. We recommend that you use gcc
instead of the HP-UX native
compiler, because gcc
produces better code!
We recommend one to use gcc 2.95 on HP-UX. Don't use high optimization flags (like -O6) as this may not be safe on HP-UX.
Note that MIT-pthreads can't be compiled with the HP-UX compiler,
because it can't compile .S
(assembler) files.
The following configure line should work:
CFLAGS="-DHPUX -I/opt/dce/include" CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions -fno-rtti" CXX=gcc ./configure --with-pthread --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared
If you are compiling gcc
2.95 yourself, you should NOT link it with
the DCE libraries (libdce.a
or libcma.a
) if you want to compile
MySQL with MIT-pthreads. If you mix the DCE and MIT-pthreads
packages you will get a mysqld
to which you cannot connect. Remove
the DCE libraries while you compile gcc
2.95!
This section describes installation and use of MySQL on Win32. This is also described in the `README' file that comes with the MySQL Win32 distribution.
If you don't have a registered version of MySQL, you should first download the shareware version from:
If you plan to connect to MySQL from some other program, you will probably also need the MyODBC driver. You can find this at the MySQL download page.
To install either distribution, unzip it in some empty directory and run the
Setup.exe
program.
By default, MySQL-Win32 is configured to be installed in
`C:\mysql'. If you want to install MySQL elsewhere, install it
in `C:\mysql', then move the installation to where you want it. If you
do move MySQL, you must tell mysqld
where everything is by
supplying options to mysqld
. Use C:\mysql\bin\mysqld --help
to
display all options! For example, if you have moved the MySQL
distribution to `D:\programs\mysql', you must start mysqld
with:
D:\programs\mysql\bin\mysqld --basedir D:\programs\mysql
With the registered version of MySQL, you can also create a
`C:\my.cnf' file that holds any default options for the
MySQL server. Copy the file `\mysql\my-example.cnf' to
`C:\my.cnf' and edit this to suit your setup. Note that you should
specify all paths with /
instead of \
. If you use
\
, you need to specify this twice, as \
is the escape
character in MySQL.
See section 4.15.4 Option files.
MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server). Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.
Note that if you are using an old Win95 release (for example OSR2), it's likely that you have an old Winsock package! MySQL requires Winsock 2! You can get the newest Winsock from Microsoft. Win98 has as default the new Winsock 2 library, so the above doesn't apply for Win98.
There are 2 different MySQL servers you can use:
mysqld | Compiled with full debugging and automatic memory allocation checking |
mysqld-opt | Optimized for a Pentium processor. |
Both of the above should work on any Intel processor >= i386.
To start the mysqld
server, you should start a MS-DOS window and type:
C:\mysql\bin\mysqld
This will start mysqld
in the background without a window.
You can kill the MySQL server by executing:
C:\mysql\bin\mysqladmin -u root shutdown
Note that Win95/Win98 don't support creation of named pipes. On Win95/Win98, you can only use named pipes to connect to a remote MySQL running on an NT server.
The Win95/Win98 section also applies to MySQL on NT, with the following differences:
To get MySQL to work with TCP/IP, you must install service pack 3 (or newer)!
For NT, the server name is mysqld-nt
. Normally you should install
MySQL as a service on NT:
C:\mysql\bin\mysqld-nt --install
(You could use the mysqld
or mysqld-opt
servers on NT,
but those cannot be started as a service or use named pipes.)
You can start and stop the MySQL service with:
NET START mysql NET STOP mysql
Note that in this case you can't use any other options for mysqld-nt
!
You can also run mysqld-nt
as a standalone program on NT if you need
to start mysqld-nt
with any options! If you start mysqld-nt
without options on NT, mysqld-nt
tries to starts itself as a service
with the default service options. If you have stopped mysqld-nt
, you
have to start it with NET START mysql
.
The service is installed with the name MySql
. Once installed, it must
be started using Services Control Manager (SCM) Utility (found in Control
Panel) or by using the NET START MySQL
command. If any options are
desired, they must be specified as "Startup parameters" in the SCM utility
before you start the MySQL service. Once running, mysqld-nt
can be stopped using mysqladmin
or from the SCM utility or by using
the command NET STOP MySQL
. If you use SCM to stop mysqld-nt
,
there is a strange message from SCM about mysqld shutdown normally
.
When run as a service, mysqld-nt
has no access to a console and so no
messages can be seen.
On NT you can get the following service error messages:
Permission Denied | Means that it cannot find mysqld-nt.exe
|
Cannot Register | Means that the path is incorrect |
If you have problems installing mysqld-nt
as a service, try starting
it with the full path:
C:\mysql\bin\mysqld --install
If this doesn't work, you can get mysqld-nt
to start properly by fixing
the path in the registry!
If you don't want to start mysqld-nt
as a service, you can start it as
follows:
C:\mysql\bin\mysqld-nt --standalone
or
C:\mysql\bin\mysqld-nt --standalone --debug
The last version gives you a debug trace in `C:\mysqld.trace'.
MySQL supports TCP/IP on all Win32 platforms and named pipes on NT. The default is to use named pipes for local connections on NT and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used:
protocol | |
NULL (none) | On NT, try named pipes first; if that doesn't work, use TCP/IP. On Win95/Win98, TCP/IP is used. |
. | Named pipes |
localhost | TCP/IP to current host |
hostname | TCP/IP |
You can force a MySQL client to use named pipes by specifying the
--pipe
option. Use the --socket
option to specify the name of
the pipe.
You can test whether or not MySQL is working by executing the following commands:
C:\mysql\bin\mysqlshow C:\mysql\bin\mysqlshow -u root mysql C:\mysql\bin\mysqladmin version status proc C:\mysql\bin\mysql test
If mysqld
is slow to answer to connections on Win95/Win98, there is
probably a problem with your DNS. In this case, start mysqld
with
--skip-name-resolve
and use only localhost
and IP numbers in
the MySQL grant tables. You can also avoid DNS when connecting to a
mysqld-nt
MySQL server running on NT by using the
--pipe
argument to specify use of named pipes. This works for most
MySQL clients.
There are two versions of the MySQL command line tool:
mysql | Compiled on native Win32, which offers very limited text editing capabilities. |
mysqlc | Compiled with the Cygnus GNU compiler and libraries, which offers readline editing.
|
If you want to use mysqlc.exe
, you must copy
`C:\mysql\lib\cygwinb19.dll' to `\windows\system' (or similar
place).
The default privileges on Win32 give all local users full privileges
to all databases. To make MySQL more secure, you
should set a password for all users and remove the row in the
mysql.user
table that has Host='localhost'
and
User=''
.
You should also add a password for the root
user:
(The following example starts by removing the anonymous user, that allows
anyone to access the 'test' database)
C:\mysql\bin\mysql mysql mysql> DELETE FROM user WHERE Host='localhost' AND User=''; mysql> QUIT C:\mysql\bin\mysqladmin reload C:\mysql\bin\mysqladmin -u root password your_password
After you've set the password, if you want to take down the mysqld
server, you can do so using this command:
mysqladmin --user=root --password=your_password shutdown
If you are using the shareware version of MySQL under windows,
the above command will fail with an error: parse error near 'SET
OPTION password'
. This is because the shareware version, which is
based on MySQL 3.21, doesn't have the SET PASSWORD
command.
With the shareware version you can set the password for the root user as follows:
C:\mysql\bin\mysql mysql mysql> UPDATE user SET password=PASSWORD('your password') WHERE user='root'; mysql> QUIT C:\mysql\bin\mysqladmin reload
With the registered MySQL version you can easily add new users
and change privileges with GRANT
and REVOKE
commands.
See section 7.26 GRANT
and REVOKE
syntax. With the Windows shareware version on has to use
INSERT
, UPDATE
and DELETE
one the tables in the
mysql
database to manage users and their privileges.
See section 6.13 Causes of Access denied
errors.
Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson).
local port: 3306
,
host: localhost
, remote port: 3306
That's it. It works very well with a direct Internet connection. I'm having problems with SSH conflicting with my Win95 network and Wingate - but that'll be the topic of a posting on another software company's usegroup!
MySQL-Win32 has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions:
mysqld
for an extended time on Win95 if
you do many connections, since each connection in MySQL creates
a new thread! WinNT and Win98 don't suffer from this bug.
mysqladmin kill
will not work on a sleeping connection.
mysqladmin shutdown
can't abort as long as there are sleeping
connections.
DROP DATABASE
mysqladmin shutdown
.
my_table
and as MY_TABLE
:
SELECT * FROM my_table WHERE MY_TABLE.col=1;
LOAD
DATA INFILE
or SELECT ... INTO OUTFILE
, you must double the `\'
character or use Unix style filenames `/' characters:
LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr; SELECT * FROM skr INTO OUTFILE 'C:/tmp/skr.txt';
Can't open named pipe
error
error 2017: can't open named pipe to host: . pipe...This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the
--host=localhost
option to the new MySQL clients
or create a file `C:\my.cnf' that contains the following information:
[client] host = localhost
Access denied for user
error
Access denied for user: 'some-user@unknown'
to database 'mysql'
when accessing a MySQL server on the same
machine, this means that MySQL can't resolve your host name
properly.
To fix this, you should create a file `\windows\hosts' with the
following information:
127.0.0.1 localhost
Here are some open issues for anyone who might want to help us with the Win32 release:
MYSQL.DLL
server. This should include everything in
a standard MySQL server, except thread creation. This will make
MySQL much easier to use in applications that don't need a true
client/server and don't need to access the server from other hosts.
mysqld.cc
,
but it should be recoded to be more ``parameter'' oriented.
The tool should also be able to update the `\my.cnf' file if the user
would prefer to use this instead of the registry.
mysqld
as a service with --install
(on NT)
it would be nice if you could also add default options on the command line.
For the moment, the workaround is to update the `C:\my.cnf' file
instead.
mysqld
daemon doesn't accept new connections when the laptop is resumed.
We don't know if this is a problem with Win95, TCP/IP or MySQL.
mysqld
from the
task manager. For the moment, you must use mysqladmin shutdown
.
readline
to Win32 for use in the mysql
command line tool.
mysql
,
mysqlshow
, mysqladmin
, and mysqldump
) would be nice.
mysqladmin kill
on Win32.
mysqld
always starts in the "C" locale and not in the default locale.
We would like to have mysqld
use the current locale for the sort order.
sqlclient
to Win32 (almost done) and add more features to it!
.DLL
s.
Other Win32-specific issues are described in the `README' file that comes with the MySQL-Win32 distribution.
MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file:
SET EMXOPT=-c -n -h1024
If you don't do this, you will probably run into the following error:
File 'xxxx' not found (Errcode: 24)
When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long file names such as HPFS, FAT32, etc.
The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'.
The `scripts/mysql-install-db' script has been renamed: it is now called `install.cmd' and is a REXX script which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.
Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads runtime library.
gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \ -o example udf_example.cc -L../lib -lmysqlclient udf_example.def mv example.dll example.udf
Note: Due to limitations in OS/2, UDF module name stems must not
exceed 8 characters. Modules are stored in the `/mysql2/udf'
directory; the safe-mysqld.cmd
script will put this directory in
the BEGINLIBPATH
environment variable. When using UDF modules,
specified extensions are ignored -- it is assumed to be `.udf'.
For example, in Unix, the shared module might be named `example.so'
and you would load a function from it like this:
CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so";
Is OS/2, the module would be named `example.udf', but you would not specify the module extension:
CREATE FUNCTION metaphon RETURNS STRING SONAME "example";
As a service, TcX provides a set of binary distributions of MySQL that are compiled at TcX or at sites where customers kindly have given us access to their machines.
These distributions
are generated with scripts/make_binary_distribution
and are
configured with the following compilers and options:
gcc
2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --disable-shared
egcs
1.0.3a
CC=gcc CFLAGS="-O6 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory
egcs
2.90.27
CC=gcc CFLAGS="-O6 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory
gcc
2.8.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory
pgcc
2.90.29 (egcs
1.0.3a)
CFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static
gcc
2.7-95q4
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
gcc
2.7.2.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
gcc
2.8.1
CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory
gcc
2.8.0
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
gcc
2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql
gcc
2.7.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
Anyone who has more optimal options for any of the configurations listed above can always mail them to the developer's mailing list at
RPM distributions prior to MySQL 3.22 are user-contributed. Beginning with 3.22, some RPMs are TcX-generated.
Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.
Normally you install the grant tables and start the server like this for installation from a source distribution:
shell> ./scripts/mysql_install_db shell> cd mysql_installation_directory shell> ./bin/safe_mysqld &
For a binary distribution, do this:
shell> cd mysql_installation_directory shell> ./bin/mysql_install_db shell> ./bin/safe_mysqld &
Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree.
In the commands shown below in this section and in the following
subsections, BINDIR
is the path to the location in which programs
like mysqladmin
and safe_mysqld
are installed. For a
binary distribution, this is the `bin' directory within the
distribution. For a source distribution, BINDIR
is probably
`/usr/local/bin', unless you specified an installation directory
other than `/usr/local' when you ran configure
.
EXECDIR
is the location in which the mysqld
server is
installed. For a binary distribution, this is the same as
BINDIR
. For a source distribution, EXECDIR
is probably
`/usr/local/libexec'.
Testing is described in detail below:
mysqld
server and set up the initial
MySQL grant tables containing the privileges that determine how
users are allowed to connect to the server. This is normally done with the
mysql_install_db
script:
shell> scripts/mysql_install_dbTypically,
mysql_install_db
needs to be run only the first time you
install MySQL. Therefore, if you are upgrading an existing
installation, you can skip this step. (However, mysql_install_db
is
quite safe to use and will not update any tables that already exist, so if
you are unsure what to do, you can always run mysql_install_db
.)
mysql_install_db
creates six tables (user
, db
,
host
, tables_priv
, columns_priv
and func
) in the
mysql
database. A description of the initial privileges is given in
section 6.10 Setting up the initial MySQL privileges. Briefly, these privileges allow the MySQL
root
user to do anything, and allow anybody to create or use databases
with a name of 'test'
or starting with 'test_'
.
If you don't set up the grant tables, the following error will appear in the
log file when you start the server:
mysqld: Can't find file: 'host.frm'The above may also happens with a binary MySQL distribution if you don't start MySQL by executing exactly
./bin/safe_mysqld
!
You might need to run mysql_install_db
as root
. However,
if you prefer, you can run the MySQL server as an unprivileged
(non-root
) user, provided that user can read and write files in
the database directory. Instructions for running MySQL as an
unprivileged user are given in section 18.8 How to run MySQL as a normal user.
If you have problems with mysql_install_db
, see
section 4.15.1 Problems running mysql_install_db
.
There are some alternatives to running the mysql_install_db
script as it is provided in the MySQL distribution:
mysql_install_db
before running it, to
change the initial privileges that are installed into the grant tables.
This is useful if you want to install MySQL on a lot of machines
with the same privileges. In this case you probably should need only to add
a few extra INSERT
statements to the mysql.user
and
mysql.db
tables!
mysql_install_db
, then use mysql -u root mysql
to
connect to the grant tables as the MySQL root
user and issue
SQL statements to modify the grant tables directly.
mysql_install_db
.
shell> cd mysql_installation_directory shell> bin/safe_mysqld &If you have problems starting the server, see section 4.15.2 Problems starting the MySQL server.
mysqladmin
to verify that the server is running. The following
commands provide a simple test to check that the server is up and responding
to connections:
shell> BINDIR/mysqladmin version shell> BINDIR/mysqladmin variablesThe output from
mysqladmin version
varies slightly depending on your
platform and version of MySQL, but should be similar to that shown
below:
shell> BINDIR/mysqladmin version mysqladmin Ver 6.3 Distrib 3.22.9-beta, for pc-linux-gnu on i686 TCX Datakonsult AB, by Monty Server version 3.22.9-beta Protocol version 10 Connection Localhost via UNIX socket TCP port 3306 UNIX socket /tmp/mysql.sock Uptime: 16 sec Running threads: 1 Questions: 20 Reloads: 2 Open tables: 3To get a feeling for what else you can do with
BINDIR/mysqladmin
,
invoke it with the --help
option.
shell> BINDIR/mysqladmin -u root shutdown
safe_mysqld
or
by invoking mysqld
directly. For example:
shell> BINDIR/safe_mysqld --log &If
safe_mysqld
fails, try running it from the MySQL
installation directory (if you are not already there). If that doesn't work,
see section 4.15.2 Problems starting the MySQL server.
shell> BINDIR/mysqlshow +-----------+ | Databases | +-----------+ | mysql | +-----------+ shell> BINDIR/mysqlshow mysql Database: mysql +--------------+ | Tables | +--------------+ | columns_priv | | db | | func | | host | | tables_priv | | user | +--------------+ shell> BINDIR/mysql -e "select host,db,user from db" mysql +------+--------+------+ | host | db | user | +------+--------+------+ | % | test | | | % | test_% | | +------+--------+------+There is also a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands:
shell> cd sql-bench shell> run-all-testsIf you don't have the `sql-bench' directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, you can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this:
shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tstThe expected results are shown in the `./tests/auto_increment.res' file.
mysql_install_db
This section lists problems you might encounter when you run
mysql_install_db
:
mysql_install_db
doesn't install the grant tables
mysql_install_db
fails to install the grant
tables and terminates after displaying the following messages:
starting mysqld daemon with databases from XXXXXX mysql daemon endedIn this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why
mysqld
didn't start. If you don't understand
what happened, include the log when you post a bug report using
mysqlbug
!
See section 2.3 How to report bugs or problems.
mysqld
daemon running
mysql_install_db
at
all. You have to run mysql_install_db
only once, when you install
MySQL the first time.
mysqld
daemon doesn't work when one daemon is running
Can't start server: Bind on
TCP/IP port: Address already in use
or Can't start server : Bind on
unix socket...
You can start the new server with a different socket and
port as follows:
shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock shell> MYSQL_TCP_PORT=3307 shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT shell> scripts/mysql_install_db shell> bin/safe_mysqld &After this, you should edit your server boot script to start both daemons with different sockets and ports. For example, it could invoke
safe_mysqld
twice, but with different --socket
, --port
and --basedir
options for each invocation.
mysql_install_db
or when
starting or using mysqld
.
You can specify a different socket and temporary directory as follows:
shell> TMPDIR=/some_tmp_dir/ shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock shell> export TMPDIR MYSQL_UNIX_PORT`some_tmp_dir' should be the path to some directory for which you have write permission. After this you should be able to run
mysql_install_db
and start
the server with these commands:
shell> scripts/mysql_install_db shell> BINDIR/safe_mysqld &
mysqld
crashes immediately
glibc
older than
2.0.7-5, you should make sure you have installed all glibc
patches!
There is a lot of information about this in the MySQL mail
archives. Links to the mail archives are available at the online
MySQL documentation page.
Also, see section 4.11.5 Linux notes (all Linux versions).
You can also start mysqld
manually using the --skip-grant
option and add the privilege information yourself using mysql
:
shell> BINDIR/safe_mysqld --skip-grant & shell> BINDIR/mysql -u root mysqlFrom
mysql
, manually execute the SQL commands in mysql_install_db
.
Make sure you run mysqladmin reload
afterward to tell the
server to reload the grant tables.
Generally, you start the mysqld
server in one of three ways:
mysql.server
. This script is used primarily at
system startup and shutdown, and is described more fully in
section 4.15.3 Starting and stopping MySQL automatically.
safe_mysqld
, which tries to determine the proper options
for mysqld
and then runs it with those options.
mysqld
directly.
Whichever method you use to start the server, if it fails to start up
correctly, check the log file to see if you can find out why. Log files
are located in the data directory (typically
`/usr/local/mysql/data' for a binary distribution,
`/usr/local/var' for a source distribution). Look in the data
directory for files with names of the form `host_name.err' and
`host_name.log' where host_name
is the name of your server
host. Then check the last few lines of these files:
shell> tail host_name.err shell> tail host_name.log
When the mysqld
daemon starts up, it changes directory to the
data directory. This is where it expects to write log files and the pid
(process ID) file, and where it expects to find databases.
The data directory location is hardwired in when the distribution is
compiled. However, if mysqld
expects to find the data directory
somewhere other than where it really is on your system, it will not work
properly. If you have problems with incorrect paths, you can find out
what options mysqld
allows and what the default path settings are by
invoking mysqld
with the --help
option. You can override the
defaults by specifying the correct pathnames as command-line arguments to
mysqld
. (These options can be used with safe_mysqld
as well.)
Normally you should need to tell mysqld
only the base directory under
which MySQL is installed. You can do this with the --basedir
option. You can also use --help
to check the effect of changing path
options (note that --help
must be the final option of the
mysqld
command). For example:
shell> EXECDIR/mysqld --basedir=/usr/local --help
Once you determine the path settings you want, start the server without
the --help
option.
If you get the following error, it means that some other program (or another
mysqld
server) is already using the TCP/IP port or socket
mysqld
is trying to use:
Can't start server: Bind on TCP/IP port: Address already in use or Can't start server : Bind on unix socket...
Use ps
to make sure that you don't have another mysqld
server
running. If you can't find another server running, you can try to execute
the command telnet your-host-name tcp-ip-port-number
and press
RETURN
a couple of times. If you don't get a error message like
telnet: Unable to connect to remote host: Connection refused
,
something is using the TCP/IP port mysqld
is trying to use.
See section 4.15.1 Problems running mysql_install_db
, and section 19.3 Running multiple MySQL servers on the same machine.
The safe_mysqld
script is written so that it normally is able to start
a server that was installed from either a source or a binary version of
MySQL, even if these install the server in slightly different
locations. safe_mysqld
expects one of these conditions to be true:
safe_mysqld
is invoked. safe_mysqld
looks under its working
directory for `bin' and `data' directories (for binary
distributions) or for `libexec' and `var' directories (for source
distributions). This condition should be met if you execute
safe_mysqld
from your MySQL installation directory (for
example, `/usr/local/mysql' for a binary distribution).
safe_mysqld
attempts to locate them by absolute pathnames. Typical
locations are `/usr/local/libexec' and `/usr/local/var'.
The actual locations are determined when the distribution was built from which
safe_mysqld
comes. They should be correct if
MySQL was installed in a standard location.
Since safe_mysqld
will try to find the server and databases relative
to its own working directory, you can install a binary distribution of
MySQL anywhere, as long as you start safe_mysqld
from the
MySQL installation directory:
shell> cd mysql_installation_directory shell> bin/safe_mysqld &
If safe_mysqld
fails, even when invoked from the MySQL
installation directory, you can modify it to use the path to mysqld
and the pathname options that are correct for your system. Note that if you
upgrade MySQL in the future, your modified version of
safe_mysqld
will be overwritten, so you should make a copy of your
edited version that you can reinstall.
If mysqld
is currently running, you can find out what path settings
it is using by executing this command:
shell> mysqladmin variables or shell> mysqladmin -h 'your-host-name' variables
If safe_mysqld
starts the server but you can't connect to it,
you should make sure you have an entry in `/etc/hosts' that looks like
this:
127.0.0.1 localhost
This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.
The mysql.server
script can be used to start or stop the server,
by invoking it with start
or stop
arguments:
shell> mysql.server start shell> mysql.server stop
mysql.server
can be found in the `share/mysql' directory
under the MySQL installation directory, or in the `support-files'
directory of the MySQL source tree.
Before mysql.server
starts the server, it changes directory to
the MySQL installation directory, then invokes
safe_mysqld
. You might need to edit mysql.server
if you
have a binary distribution that you've installed in a non-standard
location. Modify it to cd
into the proper directory before it
runs safe_mysqld
. If you want the server to run as some specific
user, you can change the mysql_daemon_user=root
line to use
another user. You can also modify mysql.server
to pass other
options to safe_mysqld
.
mysql.server stop
brings down server by sending a signal to it.
You can take down the server manually by executing mysqladmin shutdown
.
You might want to add these start and stop commands to the appropriate places
in your `/etc/rc*' files when you start using MySQL for
production applications. Note that if you modify mysql.server
, then
if you upgrade MySQL sometime, your modified version will be
overwritten, so you should make a copy of your edited version that you can
reinstall.
If your system uses `/etc/rc.local' to start external scripts, you should append the following to it:
/bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld &'
You can also add options for mysql.server
in a global
`/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like
this:
[mysqld] datadir=/usr/local/mysql/var socket=/tmp/mysqld.sock port=3306 [mysql.server] user=mysql basedir=/usr/local/mysql
The mysql.server
script uses the following variables:
user
, datadir
, basedir
, bindir
and pid-file
.
See section 4.15.4 Option files.
MySQL 3.22 can read default startup options for the server and for clients from option files.
MySQL reads default options from the following files on Unix:
Filename | Purpose |
/etc/my.cnf | Global options |
DATADIR/my.cnf | Server-specific options |
~/.my.cnf | User-specific options |
DATADIR
is the MySQL data directory (typically
`/usr/local/mysql/data' for a binary installation, or
`/usr/local/var' for a source installation). Note that this is the
directory that was specified at configuration time, not the one specified
with --datadir
when mysqld
starts up! (--datadir
has no
effect on where the server looks for option files, because it looks for them
before it processes any command-line arguments.)
MySQL reads default options from the following files on Win32:
Filename | Purpose |
windows-system-directory\my.ini
| |
C:\my.cnf | Global options |
C:\mysql\data\my.cnf | Server-specific options |
Note that you on Win32 should specify all paths with /
instead of
\
. If you use \
, you need to specify this twice, as
\
is the escape character in MySQL.
MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files take precedence over environment variable values.
The following programs support option files: mysql
,
mysqladmin
, mysqld
, mysqldump
, mysqlimport
,
mysql.server
, myisamchk
and myisampack
.
You can use option files to specify any long option that a program supports!
Run the program with --help
to get a list of available options.
An option file can contain lines of the following forms:
#comment
[group]
group
is the name of the program or group for which you want to set
options. After a group line, any option
or set-variable
lines
apply to the named group until the end of the option file or another group
line is given.
option
--option
on the command line.
option=value
--option=value
on the command line.
set-variable = variable=value
--set-variable variable=value
on the command line.
This syntax must be used to set a mysqld
variable.
The client
group allows you to specify options that apply to all
MySQL clients (not mysqld
). This is the perfect group to use
to specify the password you use to connect to the server. (But make
sure the option file is readable and writable only to yourself.)
Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\' and `\s' in your value string (`\s' == blank).
Here is a typical global option file:
[client] port=3306 socket=/tmp/mysql.sock [mysqld] port=3306 socket=/tmp/mysql.sock set-variable = key_buffer=16M set-variable = max_allowed_packet=1M [mysqldump] quick
Here is typical user option file:
[client] # The following password will be sent to all standard MySQL clients password=my_password [mysql] no-auto-rehash
If you have a source distribution, you will find a sample configuration file
named `my-example.cnf' in the `support-files' directory. If you
have a binary distribution, look in the `DIR/share/mysql' directory,
where DIR
is the pathname to the MySQL installation directory
(typically `/usr/local/mysql'). You can copy `my-example.cnf' to
your home directory (rename the copy to `.my.cnf') to experiment with.
To tell a MySQL program not to read any option files, specify
--no-defaults
as the first option on the command line. This
MUST be the first option or it will have no effect!
If you want to check which options are used, you can give the option
--print-defaults
as the first option.
If you want to force the use of a specific config file, you can use the option
--defaults-file=full-path-to-default-file
. If you do this, only the
specified file will be read.
Note for developers: Option file handling is implemented simply by processing all matching options (i.e., options in the appropriate group) before any command line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.
You can always move the MySQL form and data files between
different versions on the same architecture as long as you have the same
base version of MySQL. The current base version is
3. If you change the character set by recompiling MySQL (which may
also change the sort order), you must run myisamchk -r -q
on all
tables. Otherwise your indexes may not be ordered correctly.
If you are paranoid and/or afraid of new versions, you can always rename your
old mysqld
to something like mysqld
-'old-version-number'. If
your new mysqld
then does something unexpected, you can simply shut it
down and restart with your old mysqld
!
When you do an upgrade you should also backup your old databases, of course. Sometimes it's good to be a little paranoid!
After an upgrade, if you experience problems with recompiled client programs,
like Commands out of sync
or unexpected core dumps, you probably have
used an old header or library file when compiling your programs. In this
case you should check the date for your `mysql.h' file and
`libmysqlclient.a' library to verify that they are from the new
MySQL distribution. If not, please recompile your programs!
If you get some problems that the new mysqld
server doesn't want to
start or that you can't connect without a password, check that you don't
have some old `my.cnf' file from your old installation! You can
check this with: program-name --print-defaults
. If this outputs
anything other than the program name, you have a active my.cnf
file that will may affect things!
It is a good idea to rebuild and reinstall the Msql-Mysql-modules
distribution whenever you install a new release of MySQL,
particularly if you notice symptoms such as all your DBI
scripts
dumping core after you upgrade MySQL.
MySQL 3.23 supports tables of the new MyISAM
type and
the old ISAM
type. You don't have to convert your old tables to
use these with 3.23. By default, all new tables will be created with
type MyISAM
(unless you start mysqld
with the
--default-table-type=isam
option. You can change an ISAM
table to a MyISAM
table with ALTER TABLE
or the Perl script
mysql_convert_table_format
.
3.22 and 3.21 clients will work without any problems with a 3.23 server.
The following lists what you have to watch out for when upgrading to 3.23:
INNER
and DELAYED
are now reserved words.
FLOAT(X)
is now a true floating point types.
DECIMAL(length,dec)
the length argument no
longer includes a place for the sign or the decimal point.
TIME
string must now be of one of the following formats:
[[[DAYS] [H]H:]MM:]SS[.fraction]
or
[[[[[H]H]H]H]MM]SS[.fraction]
LIKE
now compares strings using the same character
comparison rules as '='
. If you require the old behavior, you
can compile MySQL with the CXXFLAGS=-DLIKE_CMP_TOUPPER
flag.
REGEXP
is now case insensitive for normal (not binary) strings.
myisamchk
for
MyISAM
tables (.MYI
) and isamchk
for ISAM
(.ISM
) tables.
mysqldump
s to be compatible between
MySQL 3.22 and 3.23, you should not use the --opt
or
--full
option to mysqldump
.
DATE_FORMAT()
to make sure there is a `%'
before each format character.
mysql_fetch_fields_direct
is now a function (it was a macro) and
it returns a pointer to a MYSQL_FIELD
instead of a
MYSQL_FIELD
.
mysql_num_fields()
can no longer be used on a MYSQL*
object (it's
now a function that takes MYSQL_RES*
as an argument. You should now
use mysql_field_count()
instead.
MySQL
3.22, the output of SELECT DISTINCT ...
was
almost always sorted. In 3.23, you must use GROUP BY
or
ORDER BY
to obtain sorted output.
SUM()
now returns NULL
, instead of 0, if there is no matching
rows. This is according to ANSI SQL.
CASE, THEN, WHEN, ELSE and END
Nothing that affects compatibility has changed between 3.21 and 3.22. The
only pitfall is that new tables that are created with DATE
type
columns will use the new way to store the date. You can't access these new
fields from an old version of mysqld
.
After installing MySQL 3.22, you should start the new server and
then run the mysql_fix_privilege_tables
script. This will add the new
privileges that you need to use the GRANT
command. If you forget
this, you will get Access denied
when you try to use ALTER
TABLE
, CREATE INDEX
or DROP INDEX
. If your MySQL root
user requires a password, you should give this as an argument to
mysql_fix_privilege_tables
.
The C API interface to mysql_real_connect()
has changed. If you have
an old client program that calls this function, you must place a 0
for
the new db
argument (or recode the client to send the db
element for faster connections). You must also call mysql_init()
before calling mysql_real_connect()
! This change was done to allow
the new mysql_options()
function to save options in the MYSQL
handler structure.
If you are running a version older than 3.20.28 and want to switch to 3.21.x, you need to do the following:
You can start the mysqld
3.21 server with safe_mysqld
--old-protocol
to use it with clients from the 3.20 distribution.
In this case, the new client function mysql_errno()
will not
return any server error, only CR_UNKNOWN_ERROR
, (but it
works for client errors) and the server uses the old password() checking
rather than the new one.
If you are NOT using the --old-protocol
option to
mysqld
, you will need to make the following changes:
scripts/add_long_password
must be run to convert the
Password
field in the mysql.user
table to CHAR(16)
.
mysql.user
table (to get 62-bit
rather than 31-bit passwords).
MySQL 3.20.28 and above can handle the new user
table format
without affecting clients. If you have a MySQL version earlier than
3.20.28, passwords will no longer work with it if you convert the user
table. So to be safe, you should first upgrade to at least 3.20.28 and then
upgrade to 3.21.x.
The new client code works with a 3.20.x mysqld
server, so
if you experience problems with 3.21.x, you can use the old 3.20.x server
without having to recompile the clients again.
If you are not using the --old-protocol
option to mysqld
,
old clients will issue the error message:
ERROR: Protocol mismatch. Server Version = 10 Client Version = 9
The new Perl DBI
/DBD
interface also supports the old
mysqlperl
interface. The only change you have to make if you use
mysqlperl
is to change the arguments to the connect()
function.
The new arguments are: host
, database
, user
,
password
(the user
and password
arguments have changed
places).
See section 20.5.2 The DBI
interface.
The following changes may affect queries in old applications:
HAVING
must now be specified before any ORDER BY
clause.
LOCATE()
have been swapped.
DATE
,
TIME
and TIMESTAMP
.
If you are using MySQL 3.23, you can copy the .frm
, the
.MYI
and the .MYD
files between different architectures
that support the same floating point format. (MySQL takes care
of any byte swapping issues).
The MySQL ISAM
data `*.ISD' and the index files
`*.ISM' files) are architecture-dependent and in some case
OS-dependent. If you want to move your applications to another machine
that has a different architecture or OS than your current machine, you
should not try to move a database by simply copying the files to the
other machine. Use mysqldump
instead.
By default, mysqldump
will create a file full of SQL statements.
You can then transfer the file to the other machine and feed it as input
to the mysql
client.
Try mysqldump --help
to see what options are available.
If you are moving the data to a newer version of MySQL, you should use
mysqldump --opt
with the newer version to get a fast, compact dump.
The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located:
shell> mysqladmin -h 'other hostname' create db_name shell> mysqldump --opt db_name \ | mysql -h 'other hostname' db_name
If you want to copy a database from a remote machine over a slow network, you can use:
shell> mysqladmin create db_name shell> mysqldump -h 'other hostname' --opt --compress db_name \ | mysql db_name
You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this:
shell> mysqldump --quick db_name | gzip > db_name.contents.gz
(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there:
shell> mysqladmin create db_name shell> gunzip < db_name.contents.gz | mysql db_name
You can also use mysqldump
and mysqlimport
to accomplish
the database transfer.
For big tables, this is much faster than simply using mysqldump
.
In the commands shown below, DUMPDIR
represents the full pathname
of the directory you use to store the output from mysqldump
.
First, create the directory for the output files and dump the database:
shell> mkdir DUMPDIR shell> mysqldump --tab=DUMPDIR db_name
Then transfer the files in the DUMPDIR
directory to some corresponding
directory on the target machine and load the files into MySQL
there:
shell> mysqladmin create db_name # create database shell> cat DUMPDIR/*.sql | mysql db_name # create tables in database shell> mysqlimport db_name DUMPDIR/*.txt # load data into tables
Also, don't forget to copy the mysql
database, since that's where the
grant tables (user
, db
, host
) are stored. You may have
to run commands as the MySQL root
user on the new machine
until you have the mysql
database in place.
After you import the mysql
database on the new machine, execute
mysqladmin flush-privileges
so that the server reloads the grant table
information.
MySQL includes some extensions that you probably will not find in
other SQL databases. Be warned that if you use them, your code will not be
portable to other SQL servers. In some cases, you can write code that
includes MySQL extensions, but is still portable, by using comments
of the form /*! ... */
. In this case, MySQL will parse and
execute the code within the comment as it would any other MySQL
statement, but other SQL servers will ignore the extensions. For example:
SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...
If you add a version number after the '!'
, the syntax will only be
executed if the MySQL version is equal or newer than the used
version number:
CREATE /*!32302 TEMPORARY */ TABLE (a int);
The above means that if you have 3.23.02 or newer, then MySQL will use
the TEMPORARY
keyword.
MySQL extensions are listed below:
MEDIUMINT
, SET
, ENUM
and the
different BLOB
and TEXT
types.
AUTO_INCREMENT
, BINARY
,
UNSIGNED
and ZEROFILL
.
BINARY
attribute or use the BINARY
cast, which causes
comparisons to be done according to the ASCII order used on the
MySQL server host.
db_name.tbl_name
syntax. Some SQL servers provide
the same functionality but call this User space
.
MySQL dosen't support tablespaces like in:
create table ralph.my_table...IN my_tablespace
.
LIKE
is allowed on numeric columns.
INTO OUTFILE
and STRAIGHT_JOIN
in a SELECT
statement. See section 7.12 SELECT
syntax.
SQL_SMALL_RESULT
option in a SELECT
statement.
EXPLAIN SELECT
to get a description on how tables are joined.
INDEX
or KEY
in a CREATE TABLE
statement. See section 7.7 CREATE TABLE
syntax.
TEMPORARY
or IF NOT EXISTS
with CREATE TABLE
.
COUNT(DISTINCT list)
where 'list' is more than one element.
CHANGE col_name
, DROP col_name
or DROP INDEX
in an ALTER TABLE
statement. See section 7.8 ALTER TABLE
syntax.
IGNORE
in an ALTER TABLE
statement.
ADD
, ALTER
, DROP
or CHANGE
clauses in an ALTER TABLE
statement.
DROP TABLE
with the keywords IF EXISTS
.
DROP TABLE
statement.
LIMIT
clause of the DELETE
statement.
DELAYED
clause of the INSERT
and REPLACE
statements.
LOW_PRIORITY
clause of the INSERT
, REPLACE
,
DELETE
and UPDATE
statements.
LOAD DATA INFILE
. In many cases, this syntax is compatible with
Oracle's LOAD DATA INFILE
. See section 7.16 LOAD DATA INFILE
syntax.
OPTIMIZE TABLE
statement.
See section 7.9 OPTIMIZE TABLE
syntax.
SHOW
statement.
See section 7.21 SHOW
syntax (Get information about tables, columns,...).
SET OPTION
statement. See section 7.25 SET OPTION
syntax.
GROUP BY
part.
This gives better performance for some very specific, but quite normal
queries.
See section 7.4.13 Functions for use with GROUP BY
clauses.
||
and &&
operators to mean
logical OR and AND, as in the C programming language. In MySQL,
||
and OR
are synonyms, as are &&
and AND
.
Because of this nice syntax, MySQL doesn't support
the ANSI SQL ||
operator for string concatenation; use
CONCAT()
instead. Since CONCAT()
takes any number
of arguments, it's easy to convert use of the ||
operator to
MySQL.
CREATE DATABASE
or DROP DATABASE
.
See section 7.5 CREATE DATABASE
syntax.
%
operator is a synonym for MOD()
. That is,
N % M
is equivalent to MOD(N,M)
. %
is supported
for C programmers and for compatibility with PostgreSQL.
=
, <>
, <=
,<
, >=
,>
,
<<
, >>
, <=>
, AND
, OR
or LIKE
operators may be used in column comparisons to the left of the
FROM
in SELECT
statements. For example:
mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
LAST_INSERT_ID()
function.
See section 20.4.29 mysql_insert_id()
.
REGEXP
and NOT REGEXP
extended regular expression
operators.
CONCAT()
or CHAR()
with one argument or more than two
arguments. (In MySQL, these functions can take any number of
arguments.)
BIT_COUNT()
, CASE
, ELT()
,
FROM_DAYS()
, FORMAT()
, IF()
, PASSWORD()
,
ENCRYPT()
, md5()
, ENCODE()
, DECODE()
,
PERIOD_ADD()
, PERIOD_DIFF()
, TO_DAYS()
, or
WEEKDAY()
functions.
TRIM()
to trim substrings. ANSI SQL only supports removal
of single characters.
GROUP BY
functions STD()
, BIT_OR()
and
BIT_AND()
.
REPLACE
instead of DELETE
+ INSERT
.
See section 7.15 REPLACE
syntax.
FLUSH flush_option
statement.
:=
:
SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg FROM test_table; SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
If you start mysqld with the --ansi
option, the following behaviour
of MySQL changes.
||
is string concatenation instead of OR
.
"
will be a identifier quote character (like the MySQL `
quote character) and not a string quote character.
REAL
will be a synonym for FLOAT
instead of a synonym of
DOUBLE
.
We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently:
--
is only a comment if followed by a white space. See section 5.4.7 `--' as the start of a comment.
VARCHAR
columns, trailing spaces are removed when the value is
stored. See section E Known errors and design deficiencies in MySQL.
CHAR
columns are silently changed to VARCHAR
columns. See section 7.7.1 Silent column specification changes.
REVOKE
to revoke privileges for
a table. See section 7.26 GRANT
and REVOKE
syntax.
The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. See section F List of things we want to add to MySQL in the future (The TODO).
The following will not yet work in MySQL:
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2); SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);
However, in many cases you can rewrite the query without a sub select:
SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id; SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL
For more complicated sub queries you can create temporary tables to hold the sub query.
MySQL only supports INSERT ... SELECT ...
and
REPLACE ... SELECT ...
Independent sub-selects will be probably
be available in 3.24.0. You can now use the function IN()
in
other contexts, however.
SELECT INTO TABLE
MySQL doesn't yet support the Oracle SQL extension:
SELECT ... INTO TABLE ...
. MySQL supports instead the
ANSI SQL syntax INSERT INTO ... SELECT ...
, which is basically
the same thing.
Alternatively, you can use SELECT INTO OUTFILE...
or CREATE
TABLE ... SELECT
to solve your problem.
Transactions are not supported. MySQL shortly will support atomic
operations, which are like transactions without rollback. With atomic
operations, you can execute a group of INSERT
/SELECT
/whatever
commands and be guaranteed that no other thread will interfere. In this
context, you won't usually need rollback. Currently, you can prevent
interference from other threads by using the LOCK TABLES
and
UNLOCK TABLES
commands.
See section 7.24 LOCK TABLES/UNLOCK TABLES
syntax.
A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.
A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.
The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.
To see when MySQL might get stored procedures, see section F List of things we want to add to MySQL in the future (The TODO).
Note that foreign keys in SQL are not used to join tables, but are used
mostly for checking referential integrity. If you want to get results from
multiple tables from a SELECT
statement, you do this by joining
tables!
SELECT * from table1,table2 where table1.id = table2.id;
See section 7.13 JOIN
syntax. See section 8.3.5 Using foreign keys.
The FOREIGN KEY
syntax in MySQL exists only for compatibility
with other SQL vendors' CREATE TABLE
commands; it doesn't do
anything. The FOREIGN KEY
syntax without ON DELETE ...
is
mostly used for documentation purposes. Some ODBC applications may use this
to produce automatic WHERE
clauses, but this is usually easy to
override. FOREIGN KEY
is sometimes used as a constraint check, but
this check is unnecessary in practice if rows are inserted into the tables in
the right order. MySQL only supports these clauses because some
applications require them to exist (regardless of whether or not they
work!).
In MySQL, you can work around the problem of ON DELETE
...
not being implemented by adding the appropriate DELETE
statement to
an application when you delete records from a table that has a foreign key.
In practice this is as quick (in some cases quicker) and much more portable
than using foreign keys.
In the near future we will extend the FOREIGN KEY
implementation so
that at least the information will be saved in the table specification file
and may be retrieved by mysqldump
and ODBC.
There are so many problems with FOREIGN KEY
s that we don't
know where to start:
INSERT
and UPDATE
statements,
and in this case almost all FOREIGN KEY
checks are useless because you
usually insert records in the right tables in the right order, anyway.
The only nice aspect of FOREIGN KEY
is that it gives ODBC and some
other client programs the ability to see how a table is connected and to use
this to show connection diagrams and to help in building applicatons.
MySQL will soon store FOREIGN KEY
definitions so that
a client can ask for and receive an answer how the original connection was
made. The current `.frm' file format does not have any place for it.
MySQL doesn't support views, but this is on the TODO.
Some other SQL databases use `--' to start comments. MySQL
has `#' as the start comment character, even if the mysql
command line tool removes all lines that start with `--'.
You can also use the C comment style /* this is a comment */
with
MySQL.
See section 7.29 Comment syntax.
MySQL 3.23.3 and above supports the `--' comment style
only if the comment is followed by a space. This is because this
degenerate comment style has caused many problems with automatically
generated SQL queries that have used something like the following code,
where we automatically insert the value of the payment for
!payment!
:
UPDATE tbl_name SET credit=credit-!payment!
What do you think will happen when the value of payment
is negative?
Because 1--1
is legal in SQL, we think it is terrible that
`--' means start comment.
In MySQL 3.23 you can however use: 1-- This is a comment
The following discussing only concerns you if you are running an MySQL version earlier than 3.23:
If you have a SQL program in a text file that contains `--' comments you should use:
shell> replace " --" " #" < text-file-with-funny-comments.sql \ | mysql database
instead of the usual:
shell> mysql database < text-file-with-funny-comments.sql
You can also edit the command file ``in place'' to change the `--' comments to `#' comments:
shell> replace " --" " #" -- text-file-with-funny-comments.sql
Change them back with this command:
shell> replace " #" " --" -- text-file-with-funny-comments.sql
Entry level SQL92. ODBC level 0-2.
COMMIT
/ROLLBACK
MySQL doesn't support COMMIT
-ROLLBACK.
The problem is
that handling COMMIT
-ROLLBACK
efficiently would require a
completely different table layout than MySQL uses today.
MySQL would also need extra threads that do automatic cleanups on
the tables and the disk usage would be much higher. This would make
MySQL about 2-4 times slower than it is today. MySQL is
much faster than almost all other SQL databases (typically at least 2-3 times
faster). One of the reasons for this is the lack of
COMMIT
-ROLLBACK
.
For the moment, we are much more for implementing the SQL server
language (something like stored procedures). With this you would very
seldom really need COMMIT
-ROLLBACK.
This would also give much
better performance.
Loops that need transactions normally can be coded with the help of
LOCK TABLES
, and you don't need cursors when you can update records
on the fly.
We have transactions and cursors on the TODO but not quite prioritized. If
we implement these, it will be as an option to CREATE TABLE
. That
means that COMMIT
-ROLLBACK
will work only on those tables,
so that a speed penalty will be imposed on those table only.
We at TcX have a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. (Customers with higher levels of support can alter this, so things may be reprioritized.)
The current problem is actually ROLLBACK
. Without ROLLBACK
, you
can do any kind of COMMIT
action with LOCK TABLES
. To support
ROLLBACK
, MySQL would have to be changed to store all old
records that were updated and revert everything back to the starting point if
ROLLBACK
was issued. For simple cases, this isn't that hard to do (the
current isamlog
could be used for this purpose), but it would be much
more difficult to implement ROLLBACK
for ALTER/DROP/CREATE
TABLE
.
To avoid using ROLLBACK
, you can use the following strategy:
LOCK TABLES ...
to lock all the tables you want to access.
UNLOCK TABLES
to release your locks.
This is usually a much faster method than using transactions with possible
ROLLBACK
s, although not always. The only situation this solution
doesn't handle is when someone kills the threads in the middle of an
update. In this case, all locks will be released but some of the updates may
not have been executed.
You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:
For example, when we are doing updates to some customer information, we
update only the customer data that have changed and test only that none of
the changed data, or data that depend on the changed data, have changed
compared to the original row. The test for changed data is done with the
WHERE
clause in the UPDATE
statement. If the record wasn't
updated, we give the client a message: "Some of the data you have changed
have been changed by another user". Then we show the old row versus the new
row in a window, so the user can decide which version of the customer record
he should use.
This gives us something that is similar to ``column locking'' but is actually
even better, because we only update some of the columns, using values that
are relative to their current values. This means that typical UPDATE
statements look something like these:
UPDATE tablename SET pay_back=pay_back+'relative change'; UPDATE customer SET customer_date='current_date', address='new address', phone='new phone', money_he_owes_us=money_he_owes_us+'new_money' WHERE customer_id=id AND address='old address' AND phone='old phone';
As you can see, this is very efficient and works even if another client has
changed the values in the pay_back
or money_he_owes_us
columns.
In many cases, users have wanted ROLLBACK
and/or LOCK
TABLES
for the purpose of managing unique identifiers for some tables. This
can be handled much more efficiently by using an AUTO_INCREMENT
column
and either the SQL function LAST_INSERT_ID()
or the C API function
mysql_insert_id()
. See section 20.4.29 mysql_insert_id()
.
At TcX, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this:
UPDATE tbl_name SET row_flag=1 WHERE id=ID;
MySQL returns 1 for the number of affected rows if the row was
found and row_flag
wasn't already 1 in the original row.
You can think of it as MySQL changed the above query to:
UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;
MySQL has an advanced but non-standard security/privilege system. This section describes how it works.
The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with select, insert, update and delete privileges on a database.
Additional functionality includes the ability to have an anonymous user and
to grant privileges for MySQL-specific functions such as LOAD
DATA INFILE
and administrative operations.
There are several distinctions between the way user names and passwords are used by MySQL, and the way they are used by Unix or Windows:
-u
or
--user
options. This means that you can't make a database secure in
any way unless all MySQL user names have passwords. Anyone may
attempt to connect to the server using any name, and they will succeed if
they specify any name that doesn't have a password.
PASSWORD()
and ENCRYPT()
functions in section 7.4.12 Miscellaneous functions.
MySQL client programs generally require that you specify connection
parameters when you want to access a MySQL server: the host you want
to connect to, your user name and your password. For example, the
mysql
client can be started like this (optional arguments are enclosed
between `[' and `]'):
shell> mysql [-h host_name] [-u user_name] [-pyour_pass]
Alternate forms of the -h
, -u
and -p
options are
--host=host_name
, --user=user_name
and
--password=your_pass
. Note that there is no space between
-p
or --password=
and the password following it.
Note: Specifing a password on the command line is not secure!
Any user on your system may then find out your password by typing a command
like: ps auxww
. See section 4.15.4 Option files.
mysql
uses default values for connection parameters that are missing
from the command line:
localhost
.
-p
is missing.
Thus, for a Unix user joe
, the following commands are equivalent:
shell> mysql -h localhost -u joe shell> mysql -h localhost shell> mysql -u joe shell> mysql
Other MySQL clients behave similarly.
On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:
[client]
section of the
`.my.cnf' configuration file in your home directory. The relevant
section of the file might look like this:
[client] host=host_name user=user_name password=your_passSee section 4.15.4 Option files.
MYSQL_HOST
. The MySQL user name can be
specified using USER
(this is for Windows only). The password can be
specified using MYSQL_PWD
(but this is insecure; see next section).
If connection parameters are specified in multiple ways, values specified on the command line take precedence over values specified in configuration files and environment variables, and values in configuration files take precedence over values in environment variables.
It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:
-pyour_pass
or --password=your_pass
option on the command
line. This is convenient but insecure, since your password becomes visible
to system status programs (such as ps
) that may be invoked by other
users to display command lines. (MySQL clients typically overwrite
the command line argument with zeroes during their initialization sequence,
but there is still a brief interval during which the value is visible.)
-p
or --password
option (with no your_pass
value
specified). In this case, the client program solicits the password from
the terminal:
shell> mysql -u user_name -p Enter password: ********The client echoes `*' characters to the terminal as you enter your password so that onlookers cannot see it. It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs non-interactively, there is no opportunity to enter the password from the terminal.
[client]
section of the `.my.cnf' file in your
home directory:
[client] password=your_passIf you store your password in `.my.cnf', the file should not be group or world readable or writable. Make sure the file's access mode is
400
or 600
.
See section 4.15.4 Option files.
MYSQL_PWD
environment variable, but
this method must be considered extremely insecure and should not be used.
Some versions of ps
include an option to display the environment of
running processes; your password will be in plain sight for all to see if
you set MYSQL_PWD
. Even on systems without such a version of
ps
, it is unwise to assume there is no other method to observe process
environments.
All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly-protected `.my.cnf' file.
Privilege information is stored in the user
, db
, host
,
tables_priv
and columns_priv
tables in the mysql
database (that is, in the database named mysql
). The MySQL
server reads the contents of these tables when it starts up and under the
circumstances indicated in section 6.9 When privilege changes take effect.
The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:
Privilege | Column | Context |
select | Select_priv | tables |
insert | Insert_priv | tables |
update | Update_priv | tables |
delete | Delete_priv | tables |
index | Index_priv | tables |
alter | Alter_priv | tables |
create | Create_priv | databases, tables or indexes |
drop | Drop_priv | databases or tables |
grant | Grant_priv | databases or tables |
references | References_priv | databases or tables |
reload | Reload_priv | server administration |
shutdown | Shutdown_priv | server administration |
process | Process_priv | server administration |
file | File_priv | file access on server |
The select, insert, update and delete privileges allow you to perform operations on rows in existing tables in a database.
SELECT
statements require the select privilege only if they
actually retrieve rows from a table. You can execute certain SELECT
statements even without permission to access any of the databases on the
server. For example, you could use the mysql
client as a simple
calculator:
mysql> SELECT 1+1; mysql> SELECT PI()*2;
The index privilege allows you to create or drop (remove) indexes.
The alter privilege allows you to use ALTER TABLE
.
The create and drop privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.
Note that if you grant the drop privilege for the mysql
database to a user, that user can drop the database in which the
MySQL access privileges are stored!
The grant privilege allows you to give to other users those privileges you yourself possess.
The file privilege gives you permission to read and write files on
the server using the LOAD DATA INFILE
and SELECT ... INTO
OUTFILE
statements. Any user to whom this privilege is granted can read or
write any file that the MySQL server can read or write.
The remaining privileges are used for administrative operations, which are
performed using the mysqladmin
program. The table below shows which
mysqladmin
commands each administrative privilege allows you to
execute:
Privilege | Commands permitted to privilege holders |
reload | reload , refresh ,
flush-privileges ,
flush-hosts , flush-logs , flush-tables
|
shutdown | shutdown
|
process | processlist , kill
|
The reload
command tells the server to reread the grant tables. The
refresh
command flushes all tables and opens and closes the log
files. flush-privileges
is a synonym for reload
. The other
flush-*
commands perform functions similar to refresh
but are
more limited in scope, and may be preferable in some instances. For example,
if you want to flush just the log files, flush-logs
is a better choice
than refresh
.
The shutdown
command shuts down the server.
The processlist
command displays information about the threads
executing within the server. The kill
command kills server threads.
You can always display or kill your own threads, but you need the
process privilege to display or kill threads initiated by other
users.
It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:
SELECT
.
mysql
database can be used to change passwords and
other access privilege information. (Passwords are stored encrypted, so a
malicious user cannot simply read them. However, with sufficient privileges,
that same user can replace a password with a different one.)
There are some things that you cannot do with the MySQL privilege system:
The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.
MySQL considers both your hostname and user name in identifying you
because there is little reason to assume that a given user name belongs to
the same person everywhere on the Internet. For example, the user
bill
who connects from whitehouse.gov
need not be the same
person as the user bill
who connects from mosoft.com
.
MySQL handles this by allowing you to distinguish users on different
hosts that happen to have the same name: you can grant bill
one set
of privileges for connections from whitehouse.gov
, and a different set
of privileges for connections from microsoft.com
.
MySQL access control involves two stages:
The server uses the user
, db
and host
tables in the
mysql
database at both stages of access control. The fields in these
grant tables are shown below:
Table name | user | db | host
|
Scope fields | Host | Host | Host
|
User | Db | Db
| |
Password | User | ||
Privilege fields | Select_priv | Select_priv | Select_priv
|
Insert_priv | Insert_priv | Insert_priv
| |
Update_priv | Update_priv | Update_priv
| |
Delete_priv | Delete_priv | Delete_priv
| |
Index_priv | Index_priv | Index_priv
| |
Alter_priv | Alter_priv | Alter_priv
| |
Create_priv | Create_priv | Create_priv
| |
Drop_priv | Drop_priv | Drop_priv
| |
Grant_priv | Grant_priv | Grant_priv
| |
Reload_priv | |||
Shutdown_priv | |||
Process_priv | |||
File_priv |
For the second stage of access control (request verification), the server
may, if the request involves tables, additionally consult the
tables_priv
and columns_priv
tables. The fields in these
tables are shown below:
Table name | tables_priv | columns_priv
|
Scope fields | Host | Host
|
Db | Db
| |
User | User
| |
Table_name | Table_name
| |
Column_name
| ||
Privilege fields | Table_priv | Column_priv
|
Column_priv | ||
Other fields | Timestamp | Timestamp
|
Grantor |
Each grant table contains scope fields and privilege fields.
Scope fields determine the scope of each entry in the tables, i.e., the
context in which the entry applies. For example, a user
table entry
with Host
and User
values of 'thomas.loc.gov'
and
'bob'
would be used for authenticating connections made to the server
by bob
from the host thomas.loc.gov
. Similarly, a db
table entry with Host
, User
and Db
fields of
'thomas.loc.gov'
, 'bob'
and 'reports'
would be used when
bob
connects from the host thomas.loc.gov
to access the
reports
database. The tables_priv
and columns_priv
tables contain scope fields indicating tables or table/column combinations
to which each entry applies.
For access-checking purposes, comparisons of Host
values are
case insensitive. User
, Password
, Db
and
Table_name
values are case sensitive.
Column_name
values are case insensitive in MySQL 3.22.12
or later.
Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in section 6.8 Access control, stage 2: Request verification.
Scope fields are strings, declared as shown below; the default value for each is the empty string:
Field name | Type | |
Host | CHAR(60)
| |
User | CHAR(16)
| |
Password | CHAR(16)
| |
Db | CHAR(64) | (CHAR(60) for the
tables_priv and columns_priv tables)
|
In the user
, db
and host
tables,
all privilege fields are declared as ENUM('N','Y')
-- each can have a
value of 'N'
or 'Y'
, and the default value is 'N'
.
In the tables_priv
and columns_priv
tables, the privilege
fields are declared as SET
fields:
Table name | Field name | Possible set elements |
tables_priv | Table_priv | 'Select', 'Insert',
'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
|
tables_priv | Column_priv | 'Select', 'Insert',
'Update', 'References'
|
columns_priv | Column_priv | 'Select', 'Insert',
'Update', 'References'
|
Briefly, the server uses the grant tables like this:
user
table scope fields determine whether to allow or reject
incoming connections. For allowed connections, the privilege fields indicate
the user's global (superuser) privileges.
db
and host
tables are used together:
db
table scope fields determine which users can access which
databases from which hosts. The privilege fields determine which operations
are allowed.
host
table is used as an extension of the db
table when you
want a given db
table entry to apply to several hosts. For example,
if you want a user to be able to use a database from several hosts in
your network, leave the Host
value empty in the user's db
table
entry, then populate the host
table with an entry for each of those
hosts. This mechanism is described more detail in section 6.8 Access control, stage 2: Request verification.
tables_priv
and columns_priv
tables are similar to
the db
table, but are more fine-grained: they apply at the
table and column level rather than at the database level.
Note that administrative privileges (reload, shutdown,
etc.) are specified only in the user
table. This is because
administrative operations are operations on the server itself and are not
database-specific, so there is no reason to list such privileges in the
other grant tables. In fact, only the user
table need
be consulted to determine whether or not you can perform an administrative
operation.
The file privilege is specified only in the user
table, too.
It is not an administrative privilege as such, but your ability to read or
write files on the server host is independent of the database you are
accessing.
The mysqld
server reads the contents of the grant tables once, when it
starts up. Changes to the grant tables take effect as indicated in
section 6.9 When privilege changes take effect.
When you modify the contents of the grant tables, it is a good idea to make
sure that your changes set up privileges the way you want. For help in
diagnosing problems, see section 6.13 Causes of Access denied
errors. For advice on security issues,
section 6.14 How to make MySQL secure against crackers.
A useful
diagnostic tool is the mysqlaccess
script, which Yves Carlier has
provided for the MySQL distribution. Invoke mysqlaccess
with
the --help
option to find out how it works.
Note that mysqlaccess
checks access using only the user
,
db
and host
tables. It does not check table- or column-level
privileges.
When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters stage 2 and waits for requests.
Your identity is based on two pieces of information:
Identity checking is performed using the three user
table scope fields
(Host
, User
and Password
). The server accepts the
connection only if a user
table entry matches your hostname and user
name, and you supply the correct password.
Values in the user
table scope fields may be specified as follows:
Host
value may be a hostname or an IP number, or 'localhost'
to indicate the local host.
Host
field.
Host
value of '%'
matches any hostname. A blank Host
value is equivalent to '%'
. Note that these values match any
host that can create a connection to your server!
User
field, but you can
specify a blank value, which matches any name. If the user
table
entry that matches an incoming connection has a blank user name, the user is
considered to be the anonymous user (the user with no name), rather than the
name that the client actually specified. This means that a blank user name
is used for all further access checking for the duration of the connection
(that is, during stage 2).
Password
field can be blank. This does not mean that any password
matches, it means the user must connect without specifying a password.
Non-blank Password
values represent encrypted passwords.
MySQL does not store passwords in plaintext form for anyone to see.
Rather, the password supplied by a user who is attempting to connect is
encrypted (using the PASSWORD()
function) and compared to the
already-encrypted version stored in the user
table. If they match,
the password is correct.
The examples below show how various combinations of Host
and
User
values in user
table entries apply to incoming
connections:
Host value | User value | Connections matched by entry |
'thomas.loc.gov' | 'fred' | fred , connecting from thomas.loc.gov
|
'thomas.loc.gov' | '' | Any user, connecting from thomas.loc.gov
|
'%' | 'fred' | fred , connecting from any host
|
'%' | '' | Any user, connecting from any host |
'%.loc.gov' | 'fred' | fred , connecting from any host in the loc.gov domain
|
'x.y.%' | 'fred' | fred , connecting from x.y.net , x.y.com ,x.y.edu , etc. (this is probably not useful)
|
'144.155.166.177' | 'fred' | fred , connecting from the host with IP address 144.155.166.177
|
'144.155.166.%' | 'fred' | fred , connecting from any host in the 144.155.166 class C subnet
|
Since you can use IP wildcard values in the Host
field (e.g.,
'144.155.166.%'
to match every host on a subnet), there is the
possibility that someone might try to exploit this capability by naming a
host 144.155.166.somewhere.com
. To foil such attempts, MySQL
disallows matching on hostnames that start with digits and a dot. Thus, if
you have a host named something like 1.2.foo.com
, its name will never
match the Host
column of the grant tables. Only an IP number can
match an IP wildcard value.
An incoming connection may be matched by more than one entry in the
user
table. For example, a connection from thomas.loc.gov
by
fred
would be matched by several of the entries just shown above. How
does the server choose which entry to use if more than one matches? The
server resolves this question by sorting the user
table after reading
it at startup time, then looking through the entries in sorted order when a
user attempts to connect. The first matching entry is the one that is used.
user
table sorting works as follows. Suppose the user
table
looks like this:
+-----------+----------+- | Host | User | ... +-----------+----------+- | % | root | ... | % | jeffrey | ... | localhost | root | ... | localhost | | ... +-----------+----------+-
When the server reads in the table, it orders the entries with the
most-specific Host
values first ('%'
in the Host
column
means ``any host'' and is least specific). Entries with the same Host
value are ordered with the most-specific User
values first (a blank
User
value means ``any user'' and is least specific). The resulting
sorted user
table looks like this:
+-----------+----------+- | Host | User | ... +-----------+----------+- | localhost | root | ... | localhost | | ... | % | jeffrey | ... | % | root | ... +-----------+----------+-
When a connection is attempted, the server looks through the sorted entries
and uses the first match found. For a connection from localhost
by
jeffrey
, the entries with 'localhost'
in the Host
column
match first. Of those, the entry with the blank user name matches both the
connecting hostname and user name. (The '%'/'jeffrey'
entry would
have matched, too, but it is not the first match in the table.)
Here is another example. Suppose the user
table looks like this:
+----------------+----------+- | Host | User | ... +----------------+----------+- | % | jeffrey | ... | thomas.loc.gov | | ... +----------------+----------+-
The sorted table looks like this:
+----------------+----------+- | Host | User | ... +----------------+----------+- | thomas.loc.gov | | ... | % | jeffrey | ... +----------------+----------+-
A connection from thomas.loc.gov
by jeffrey
is matched by the
first entry, whereas a connection from whitehouse.gov
by
jeffrey
is matched by the second.
A common misconception is to think that for a given user name, all entries
that explicitly name that user will be used first when the server attempts to
find a match for the connection. This is simply not true. The previous
example illustrates this, where a connection from thomas.loc.gov
by
jeffrey
is first matched not by the entry containing 'jeffrey'
as the User
field value, but by the entry with no user name!
If you have problems connecting to the server, print out the user
table and sort it by hand to see where the first match is being made.
Once you establish a connection, the server enters stage 2. For each request
that comes in on the connection, the server checks whether you have
sufficient privileges to perform it, based on the type of operation you wish
to perform. This is where the privilege fields in the grant tables come into
play. These privileges can come from any of the user
, db
,
host
, tables_priv
or columns_priv
tables. The grant
tables are manipulated with GRANT
and REVOKE
commands.
See section 7.26 GRANT
and REVOKE
syntax. (You may find it helpful to refer to
section 6.6 How the privilege system works, which lists the fields present in each of the grant
tables.)
The user
table grants privileges that are assigned to you on a global
basis and that apply no matter what the current database is. For example, if
the user
table grants you the delete privilege, you can
delete rows from any database on the server host! In other words,
user
table privileges are superuser privileges. It is wise to grant
privileges in the user
table only to superusers such as server or
database administrators. For other users, you should leave the privileges
in the user
table set to 'N'
and grant privileges on a
database-specific basis only, using the db
and host
tables.
The db
and host
tables grant database-specific privileges.
Values in the scope fields may be specified as follows:
Host
and
Db
fields of either table.
'%'
Host
value in the db
table means ``any host.'' A
blank Host
value in the db
table means ``consult the
host
table for further information.''
'%'
or blank Host
value in the host
table means ``any
host.''
'%'
or blank Db
value in either table means ``any database.''
User
value in either table matches the anonymous user.
The db
and host
tables are read in and sorted when the server
starts up (at the same time that it reads the user
table). The
db
table is sorted on the Host
, Db
and User
scope
fields, and the host
table is sorted on the Host
and Db
scope fields. As with the user
table, sorting puts the most-specific
values first and least-specific values last, and when the server looks for
matching entries, it uses the first match that it finds.
The tables_priv
and columns_priv
tables grant table- and
column-specific privileges. Values in the scope fields may be specified as
follows:
Host
field of either table.
'%'
or blank Host
value in either table means ``any host.''
Db
, Table_name
and Column_name
fields cannot contain
wildcards or be blank in either table.
The tables_priv
and columns_priv
tables are sorted on
the Host
, Db
and User
fields. This is similar to
db
table sorting, although since only the Host
field may
contain wildcards, the sorting is simpler.
The request verification process is described below. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)
For administrative requests (shutdown, reload, etc.), the
server checks only the user
table entry, since that is the only table
that specifies administrative privileges. Access is granted if the entry
allows the requested operation and denied otherwise. For example, if you
want to execute mysqladmin shutdown
but your user
table entry
doesn't grant the shutdown privilege to you, access is denied
without even checking the db
or host
tables. (Since they
contain no Shutdown_priv
column, there is no need to do so.)
For database-related requests (insert, update, etc.), the
server first checks the user's global (superuser) privileges by looking in
the user
table entry. If the entry allows the requested operation,
access is granted. If the global privileges in the user
table are
insufficient, the server determines the user's database-specific privileges
by checking the db
and host
tables:
db
table for a match on the Host
,
Db
and User
fields. Host
and User
are matched to
the connecting user's hostname and MySQL user name. The Db
field is matched to the database the user wants to access. If there is no
entry for the Host
and User
, access is denied.
db
table entry and its Host
field is
not blank, that entry defines the user's database-specific privileges.
db
table entry's Host
field is blank, it
signifies that the host
table enumerates which hosts should be allowed
access to the database. In this case, a further lookup is done in the
host
table to find a match on the Host
and Db
fields.
If no host
table entry matches, access is denied. If there is a
match, the user's database-specific privileges are computed as the
intersection (not the union!) of the privileges in the db
and
host
table entries, i.e., the privileges that are 'Y'
in both
entries. (This way you can grant general privileges in the db
table
entry and then selectively restrict them on a host-by-host basis using the
host
table entries.)
After determining the database-specific privileges granted by the db
and host
table entries, the server adds them to the global privileges
granted by the user
table. If the result allows the requested
operation, access is granted. Otherwise, the server checks the user's
table and column privileges in the tables_priv
and columns_priv
tables and adds those to the user's privileges. Access is allowed or denied
based on the result.
Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:
global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges
It may not be apparent why, if the global user
entry privileges are
initially found to be insufficient for the requested operation, the server
adds those privileges to the database-, table- and column-specific privileges
later. The reason is that a request might require more than one type of
privilege. For example, if you execute an INSERT ... SELECT
statement, you need both insert and select privileges.
Your privileges might be such that the user
table entry grants one
privilege and the db
table entry grants the other. In this case, you
have the necessary privileges to perform the request, but the server cannot
tell that from either table by itself; the privileges granted by both entries
must be combined.
The host
table can be used to maintain a list of ``secure'' servers.
At TcX, the host
table contains a list of all machines on the local
network. These are granted all privileges.
You can also use the host
table to indicate hosts that are not
secure. Suppose you have a machine public.your.domain
that is located
in a public area that you do not consider secure. You can allow access to
all hosts on your network except that machine by using host
table
entries
like this:
+--------------------+----+- | Host | Db | ... +--------------------+----+- | public.your.domain | % | ... (all privileges set to 'N') | %.your.domain | % | ... (all privileges set to 'Y') +--------------------+----+-
Naturally, you should always test your entries in the grant tables (e.g.,
using mysqlaccess
) to make sure your access privileges are actually
set up the way you think they are.
When mysqld
starts, all grant table contents are read into memory and
become effective at that point.
Modifications to the grant tables that you perform using GRANT
,
REVOKE
, or SET PASSWORD
are noticed by the server immediately.
If you modify the grant tables manually (using INSERT
, UPDATE
,
etc.), you should execute a FLUSH PRIVILEGES
statement or run
mysqladmin flush-privileges
to tell the server to reload the grant
tables. Otherwise your changes will have no effect until
you restart the server.
When the server notices that the grant tables have been changed, existing client connections are affected as follows:
USE db_name
command.
Global privilege changes and password changes take effect the next time the client connects.
After installing MySQL, you set up the initial access privileges by
running scripts/mysql_install_db
.
See section 4.7.1 Quick installation overview.
The scripts/mysql_install_db
script starts up the mysqld
server, then initializes the grant tables to contain the following set
of privileges:
root
user is created as a superuser who can do
anything. Connections must be made from the local host.
Note:
The initial root
password is empty, so anyone can connect as root
without a password and be granted all privileges.
'test'
or starting with 'test_'
. Connections must be
made from the local host. This means any local user can connect and be
treated as the anonymous user.
mysqladmin shutdown
or mysqladmin processlist
.
Note: The default privileges are different for Win32. See section 4.12.4 Running MySQL on Win32.
Since your installation is initially wide open, one of the first things you
should do is specify a password for the MySQL
root
user. You can do this as follows (note that you specify the
password using the PASSWORD()
function):
shell> mysql -u root mysql mysql> UPDATE user SET Password=PASSWORD('new_password') WHERE user='root'; mysql> FLUSH PRIVILEGES;
You can in MySQL 3.22 and above use the SET PASSWORD
statement:
shell> mysql -u root mysql mysql> SET PASSWORD FOR root=PASSWORD('new_password');
Another way to set the password is by using the mysqladmin
command:
shell> mysqladmin -u root password new_password
Note that if you update the password in the user
table directly using
the first method, you must tell the server to reread the grant tables (with
FLUSH PRIVILEGES
), since the change will go unnoticed otherwise.
Once the root
password has been set, thereafter you must supply that
password when you connect to the server as root
.
You may wish to leave the root
password blank so that you don't need
to specify it while you perform additional setup or testing, but be sure to
set it before using your installation for any real production work.
See the scripts/mysql_install_db
script to see how it sets up
the default privileges. You can use this as a basis to see how to
add other users.
If you want the initial privileges to be different than those just described
above, you can modify mysql_install_db
before you run it.
To recreate the grant tables completely, remove all the `*.frm',
`*.MYI' and `*.MYD' files in the directory containing the
mysql
database. (This is the directory named `mysql' under
the database directory, which is listed when you run mysqld
--help
.) Then run the mysql_install_db
script, possibly after
editing it first to have the privileges you want.
NOTE: For MySQL versions older than 3.22.10, you should NOT
delete the `*.frm' files. If you accidentally do this, you should
copy them back from your MySQL distribution before running
mysql_install_db
.
You can add users two different ways: by using GRANT
statements
or by manipulating the MySQL grant tables directly. The
preferred method is to use GRANT
statements, because they are
more concise and less error-prone.
The examples below show how to use the mysql
client to set up new
users. These examples assume that privileges are set up according to the
defaults described in the previous section. This means that to make changes,
you must be on the same machine where mysqld
is running, you must
connect as the MySQL root
user, and the root
user must
have the insert privilege for the mysql
database and the
reload administrative privilege. Also, if you have changed the
root
user password, you must specify it for the mysql
commands
below.
You can add new users by issuing GRANT
statements:
shell> mysql --user=root mysql mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost IDENTIFIED BY 'something' WITH GRANT OPTION; mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%" IDENTIFIED BY 'something' WITH GRANT OPTION; mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost; mysql> GRANT USAGE ON *.* TO dummy@localhost;
These GRANT
statements set up three new users:
monty
'something'
to do so. Note that we must issue
GRANT
statements for both monty@localhost
and
monty@"%"
. If we don't add the entry with localhost
, the
anonymous user entry for localhost
that is created by
mysql_install_db
will take precedence when we connect from the local
host, because it has a more specific Host
field value and thuse comes
earlier in the user
table sort order.
admin
localhost
without a password and who is
granted the reload and process administrative privileges.
This allows the user to execute the mysqladmin reload
,
mysqladmin refresh
and mysqladmin flush-*
commands, as well as
mysqladmin processlist
. No database-related privileges are granted.
They can be granted later by issuing additional GRANT
statements.
dummy
'N'
-- the USAGE
privilege
type allows you to set up a user with no privileges. It is assumed that you
will grant database-specific privileges later.
You can also add the same user access information directly by issuing
INSERT
statements and then telling the server to reload the grant
tables:
shell> mysql --user=root mysql mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('something'), 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y') mysql> INSERT INTO user VALUES('%','monty',PASSWORD('something'), 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y') mysql> INSERT INTO user SET Host='localhost',User='admin', Reload_priv='Y', Process_priv='Y'; mysql> INSERT INTO user (Host,User,Password) VALUES('localhost','dummy',''); mysql> FLUSH PRIVILEGES;
Depending on your MySQL version, you may have to use a different
number of 'Y'
values above (versions prior to 3.22.11 had fewer
privilege columns). For the admin
user, the more readable extended
INSERT
syntax that is available starting with 3.22.11 is used.
Note that to set up a superuser, you need only create a user
table
entry with the privilege fields set to 'Y'
. No db
or
host
table entries are necessary.
The privilege columns in the user
table were not set explicitly in the
last INSERT
statement (for the dummy
user), so those columns
are assigned the default value of 'N'
. This is the same thing that
GRANT USAGE
does.
The following example adds a user custom
who can connect from hosts
localhost
, server.domain
and whitehouse.gov
. He wants
to access the bankaccount
database only from localhost
,
the expenses
database only from whitehouse.gov
and
the customer
database from all three hosts. He wants
to use the password stupid
from all three hosts.
To set up this user's privileges using GRANT
statements, run these
commands:
shell> mysql --user=root mysql mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON bankaccount.* TO custom@localhost IDENTIFIED BY 'stupid'; mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON expenses.* TO custom@whitehouse.gov IDENTIFIED BY 'stupid'; mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON customer.* TO custom@'%' IDENTIFIED BY 'stupid';
To set up the user's privileges by modifying the grant tables directly,
run these commands (note the FLUSH PRIVILEGES
at the end):
shell> mysql --user=root mysql mysql> INSERT INTO user (Host,User,Password) VALUES('localhost','custom',PASSWORD('stupid')); mysql> INSERT INTO user (Host,User,Password) VALUES('server.domain','custom',PASSWORD('stupid')); mysql> INSERT INTO user (Host,User,Password) VALUES('whitehouse.gov','custom',PASSWORD('stupid')); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y'); mysql> FLUSH PRIVILEGES;
The first three INSERT
statements add user
table entries that
allow user custom
to connect from the various hosts with the given
password, but grant no permissions to him (all privileges are set to the
default value of 'N'
). The next three INSERT
statements add
db
table entries that grant privileges to custom
for the
bankaccount
, expenses
and customer
databases, but only
when accessed from the proper hosts. As usual, when the grant tables are
modified directly, the server must be told to reload them (with
FLUSH PRIVILEGES
) so that the privilege changes take effect.
If you want to give a specific user access from any machine in a given
domain, you can issue a GRANT
statement like the following:
mysql> GRANT ... ON *.* TO myusername@"%.mydomainname.com" IDENTIFIED BY 'mypassword';
To do the same thing by modifying the grant tables directly, do this:
mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername', PASSWORD('mypassword'),...); mysql> FLUSH PRIVILEGES;
You can also use xmysqladmin
, mysql_webadmin
and even
xmysql
to insert, change and update values in the grant tables.
You can find these utilities at the MySQL
Contrib directory.
The examples in the preceding sections illustrate an important principle:
when you store a non-empty password using INSERT
or UPDATE
statements, you must use the PASSWORD()
function to encrypt it. This
is because the user
table stores passwords in encrypted form, not as
plaintext. If you forget that fact, you are likely to attempt to set
passwords like this:
shell> mysql -u root mysql mysql> INSERT INTO user (Host,User,Password) VALUES('%','jeffrey','biscuit'); mysql> FLUSH PRIVILEGES;
The result is that the plaintext value 'biscuit'
is stored as the
password in the user
table. When the user jeffrey
attempts to
connect to the server using this password, the mysql
client encrypts
it with PASSWORD()
and sends the result to the server. The server
compares the value in the user
table (which is the plaintext value
'biscuit'
) to the encrypted password (which is not
'biscuit'
). The comparison fails and the server rejects the
connection:
shell> mysql -u jeffrey -pbiscuit test Access denied
Since passwords must be encrypted when they are inserted in the user
table, the INSERT
statement should have been specified like this
instead:
mysql> INSERT INTO user (Host,User,Password) VALUES('%','jeffrey',PASSWORD('biscuit'));
You must also use the PASSWORD()
function when you use SET
PASSWORD
statements:
mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit');
If you set passwords using the GRANT ... IDENTIFIED BY
statement
or the mysqladmin password
command, the PASSWORD()
function
is unnecessary. They both take care of encrypting the password for you,
so you would specify a password of 'biscuit'
like this:
mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit';
or
shell> mysqladmin -u jeffrey password biscuit
Note: PASSWORD()
does not perform password encryption in the same way
that Unix passwords are encrypted. You should not assume that if your Unix
password and your MySQL password are the same, PASSWORD()
will result in the same encrypted value as is stored in the Unix password
file.
See section 6.2 MySQL user names and passwords.
Access denied
errors
If you encounter Access denied
errors when you try to connect to the
MySQL server, the list below indicates some courses of
action you can take to correct the problem:
mysql_install_db
script after installing
MySQL, to set up the initial grant table contents? If not, do
so. See section 6.10 Setting up the initial MySQL privileges. Test the initial privileges by
executing this command:
shell> mysql -u root testThe server should let you connect without error. You should also make sure you have a file `user.MYD' in the MySQL database directory. Ordinarily, this is `PATH/var/mysql/user.MYD', where
PATH
is the
pathname to the MySQL installation root.
shell> mysql -u root mysqlThe server should let you connect because the MySQL
root
user
has no password initially. Since that is also a security risk, setting the
root
password is something you should do while you're setting up
your other MySQL users.
If you try to connect as root
and get this error:
Access denied for user: '@unknown' to database mysqlthis means that you don't have an entry in the
user
table with a
User
column value of 'root'
and that mysqld
cannot
resolve the hostname for your client. In this case, you must restart the
server with the --skip-grant-tables
option and edit your
`/etc/hosts' or `\windows\hosts' file to add a entry for your
host.
mysql_fix_privilege_tables
script? If not, do so. The structure of
the grant tables changed with MySQL 3.22.11 when the GRANT
statement became functional.
INSERT
or
UPDATE
statement) and your changes seem to be ignored, remember that
you must issue a FLUSH PRIVILEGES
statement or execute a
mysqladmin flush-privileges
command to cause the server to reread the
tables. Otherwise your changes have no effect until the next time the server
is restarted. Remember that after you set the root
password, you
won't need to specify it until after you flush the privileges, because the
server still won't know you've changed the password yet!
mysqld
daemon with the
--skip-grant-tables
option. Then you can change the MySQL
grant tables and use the mysqlaccess
script to check whether or not
your modifications have the desired effect. When you are satisfied with your
changes, execute mysqladmin flush-privileges
to tell the mysqld
server to start using the new grant tables. Note: Reloading the
grant tables overrides the --skip-grant-tables
option. This allows
you to tell the server to begin using the grant tables again without bringing
it down and restarting it.
mysql -u user_name db_name
or mysql
-u user_name -pyour_pass db_name
. If you are able to connect using the
mysql
client, there is a problem with your program and not with the
access privileges. (Notice that there is no space between -p
and the
password; you can also use the --password=your_pass
syntax to specify
the password.)
PASSWORD()
function if you set the password with the
INSERT
, UPDATE
or SET PASSWORD
statements. The
PASSWORD()
function is unnecessary if you specify the password using
the GRANT ... INDENTIFIED BY
statement or the mysqladmin
password
command.
See section 6.12 How to set up passwords.
localhost
is a synonym for your local hostname, and is also the
default host to which clients try to connect if you specify no host
explicitly. However, connections to localhost
do not work if you are
running on a system that uses MIT-pthreads (localhost
connections are
made using Unix sockets, which are not supported by MIT-pthreads). To avoid
this problem on such systems, you should use the --host
option to name
the server host explicitly. This will make a TCP/IP connection to the
mysqld
server. In this case, you must have your real hostname in
user
table entries on the server host. (This is true even if you are
running a client program on the same host as the server.)
Access denied
error when trying to connect to the
database with mysql -u user_name db_name
, you may have a problem
with the user
table. Check this by executing mysql -u root
mysql
and issuing this SQL statement:
mysql> SELECT * FROM user;The result should include an entry with the
Host
and User
columns matching your computer's hostname and your MySQL user name.
Access denied
error message will tell you who you are trying
to log in as, the host from which you are trying to connect, and whether
or not you were using a password. Normally, you should have one entry in
the user
table that exactly matches the hostname and user name
that were given in the error message.
user
table that matches that host:
Host ... is not allowed to connect to this MySQL serverYou can fix this by using the command line tool
mysql
(on the server
host!) to add a row to the user
table for the user/hostname combination
from which you are trying to connect. If you are not running MySQL
3.22 and you don't know the IP number or hostname of the machine from which
you are connecting, you should put an entry with '%'
as the
Host
column value in the user
table and restart mysqld
with the --log
option on the server machine. After trying to connect
from the client machine, the information in the MySQL log will
indicate how you really did connect. (Then replace the '%'
in the
user
table entry with the actual hostname that shows up in the log.
Otherwise, you'll have a system that is insecure.)
mysql -u root test
works but mysql -h your_hostname -u root
test
results in Access denied
, then you may not have the correct name
for your host in the user
table. A common problem here is that the
Host
value in the user table entry specifies an unqualified hostname,
but your system's name resolution routines return a fully-qualified domain
name (or vice-versa). For example, if you have an entry with host
'tcx'
in the user
table, but your DNS tells MySQL that
your hostname is 'tcx.subnet.se'
, the entry will not work. Try adding
an entry to the user
table that contains the IP number of your host as
the Host
column value. (Alternatively, you could add an entry to the
user
table with a Host
value that contains a wildcard--for
example, 'tcx.%'
. However, use of hostnames ending with `%' is
insecure and is not recommended!)
mysql -u user_name test
works but mysql -u user_name
other_db_name
doesn't work, you don't have an entry for other_db_name
listed in the db
table.
mysql -u user_name db_name
works when executed on the server
machine, but mysql -u host_name -u user_name db_name
doesn't work when
executed on another client machine, you don't have the client machine listed
in the user
table or the db
table.
Access denied
, remove from the
user
table all entries that have Host
values containing
wildcards (entries that contain `%' or `_'). A very common error
is to insert a new entry with Host
='%'
and
User
='some user'
, thinking that this will allow you to specify
localhost
to connect from the same machine. The reason that this
doesn't work is that the default privileges include an entry with
Host
='localhost'
and User
=''
. Since that entry
has a Host
value 'localhost'
that is more specific than
'%'
, it is used in preference to the new entry when connecting from
localhost
! The correct procedure is to insert a second entry with
Host
='localhost'
and User
='some_user'
, or to
remove the entry with with Host
='localhost'
and
User
=''
.
db
or
host
table:
Access to database deniedIf the entry selected from the
db
table has an empty value in the
Host
column, make sure there are one or more corresponding entries in
the host
table specifying which hosts the db
table entry
applies to.
If you get the error when using the SQL commands SELECT ...
INTO OUTFILE
or LOAD DATA INFILE
, your entry in the user
table
probably doesn't have the file privilege enabled.
Access denied
when you run a client without any options, make sure you
haven't specified an old password in any of your option files!
See section 4.15.4 Option files.
mysqld
daemon with a debugging
option (for example, --debug=d,general,query
). This will print host and
user information about attempted connections, as well as information about
each command issued. See section G.1 Debugging a MySQL server.
mysqldump mysql
command. As always, post your problem using
the mysqlbug
script. In some cases you may restart mysqld
with
--skip-grant-tables
to be able to run mysqldump
.
When you connect to a MySQL server, you should normally use a password. The password is not transmitted in clear text over the connection.
All other information is transferred as text that can be read by anyone that
is able to watch the connection. If you are concerned about this, you can
use the compressed protocol (in MySQL 3.22 and above) to make things
much harder. To make things even more secure you should install ssh
(see http://www.cs.hut.fi/ssh). With this, you can get an encrypted
TCP/IP connection between a MySQL server and a MySQL
client.
To make a MySQL system secure, you should strongly consider the following suggestions:
mysql -u other_user db_name
if
other_user
has no password. It is common behavior with client/server
applications that the client may specify any user name. You can change the
password of all users by editing the mysql_install_db
script before
you run it, or only the password for the MySQL root
user like
this:
shell> mysql -u root mysql mysql> UPDATE user SET Password=PASSWORD('new_password') WHERE user='root'; mysql> FLUSH PRIVILEGES;
root
user.
mysqld
can be run as any user. You can also create a new Unix user
mysql
to make everything even more secure. If you run mysqld
as another Unix user, you don't need to change the root
user name in
the user
table, because MySQL user names have nothing to do
with Unix user names. You can edit the mysql.server
script to start
mysqld
as another Unix user. Normally this is done with the su
command. For more details, see section 18.8 How to run MySQL as a normal user.
root
user in the mysql.server
script, make sure this script is readable only by root
.
mysqld
runs as is the only user with
read/write privileges in the database directories.
mysqladmin processlist
shows the text of the currently executing
queries, so any user who is allowed to execute that command might be able to
see if another user issues an UPDATE user SET
password=PASSWORD('not_secure')
query.
mysqld
saves an extra connection for users who have the
process privilege, so that a MySQL root
user can log
in and check things even if all normal connections are in use.
mysqld
daemon! To make this a bit safer, all files generated with
SELECT ... INTO OUTFILE
are readable to everyone, and you can't
overwrite existing files.
The file privilege may also be used to read any file accessible
to the Unix user that the server runs as. This could be abused, for example,
by using LOAD DATA
to load `/etc/passwd' into a table, which
can then be read with SELECT
.
--secure
option to
mysqld
should make hostnames safe. In any case, you should be very
careful about using hostname values that contain wildcards!
The following mysqld
options affect security:
--secure
gethostbyname()
system call are checked to
make sure they resolve back to the original hostname. This makes it harder
for someone on the outside to get access by simulating another host. This
option also adds some sanity checks of hostnames. The option is turned off
by default in MySQL 3.21 since it sometimes takes a long time to
perform backward resolutions. MySQL 3.22 caches hostnames and has
this option enabled by default.
--skip-grant-tables
mysqladmin
reload
.)
--skip-name-resolve
Host
column values in the grant
tables must be IP numbers or localhost
.
--skip-networking
mysqld
must be made via Unix sockets. This option is unsuitable for
systems that use MIT-pthreads, because the MIT-pthreads package doesn't
support Unix sockets.
A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters (the later only if you don't run in ANSI mode). Examples:
'a string' "another string"
Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the escape character. MySQL recognizes the following escape sequences:
\0
NUL
) character.
\n
\t
\r
\b
\'
\"
\\
\%
\_
Note that if you use `\%' or `\%_' in some string contexts, these will return the strings `\%' and `\_' and not `%' and `_'.
There are several ways to include quotes within a string:
The SELECT
statements shown below demonstrate how quoting and
escaping work:
mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello'; +-------+---------+-----------+--------+--------+ | hello | "hello" | ""hello"" | hel'lo | 'hello | +-------+---------+-----------+--------+--------+ mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello"; +-------+---------+-----------+--------+--------+ | hello | 'hello' | ''hello'' | hel"lo | "hello | +-------+---------+-----------+--------+--------+ mysql> SELECT "This\nIs\nFour\nlines"; +--------------------+ | This Is Four lines | +--------------------+
If you want to insert binary data into a BLOB
column, the following
characters must be represented by escape sequences:
NUL
\
'
"
If you write C code, you can use the C API function
mysql_escape_string()
to escape characters for the INSERT
statement. See section 20.3 C API function overview. In Perl, you can use the
quote
method of the DBI
package to convert special
characters to the proper escape sequences. See section 20.5.2 The DBI
interface.
You should use an escape function on any string that might contain any of the special characters listed above!
Integers are represented as a sequence of digits. Floats use `.' as a decimal separator. Either type of number may be preceded by `-' to indicate a negative value.
Examples of valid integers:
1221 0 -32
Examples of valid floating-point numbers:
294.42 -32032.6809e+10 148.00
An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.
MySQL supports hexadecimal values. In number context these acts like an integer (64 bit precision). In string context these acts like a binary string where each pair of hex digits is converted to a character.
mysql> SELECT 0xa+0 -> 10 mysql> select 0x5061756c; -> Paul
Hexadecimal strings is often used by ODBC to give values for BLOB columns.
NULL
values
The NULL
value means ``no data'' and is different from values such
as 0
for numeric types or the empty string for string types.
See section 18.15 Problems with NULL
values.
NULL
may be represented by \N
when using the text file import
or export formats (LOAD DATA INFILE
, SELECT ... INTO OUTFILE
).
See section 7.16 LOAD DATA INFILE
syntax.
Database, table, index, column and alias names all follow the same rules in MySQL:
Note that the rules changed starting with MySQL 3.23.6 when we introduced
quoting of identifiers (database, table and column names) with `
("
will also work to quote identifiers if you run in ANSI mode).
Identifier | max length | Allowed characters |
Database | 64 | Any character that is allowed in a directory name execpt / .
|
Table | 64 | Any character that is allowed in file name, execpt / or .
|
Column | 64 | All characters |
Alias | 255 | All characters |
Note that in addition to the above, you can't have ASCII(0) or ASCII(255) in an identifier.
Note that if the identifer is a restricted word or contains special character
you must always quote it with `
when you use it:
SELECT * from `select` where `select`.id > 100;
In previous versions of MySQL, the name rules are as follows:
It is recommended that you do not use names like 1e
, because
an expression like 1e+1
is ambiguous. It may be interpreted as the
expression 1e + 1
or as the number 1e+1
.
In MySQL you can refer to a column using any of the following forms:
Column reference | Meaning |
col_name | Column col_name
from whichever table used in the query contains a column of that name
|
tbl_name.col_name | Column col_name from table
tbl_name of the current database
|
db_name.tbl_name.col_name | Column col_name from table
tbl_name of the database db_name . This form is available in
MySQL 3.22 or later.
|
`column_name` | A column that is a keyword or contains special characters. |
You need not specify a tbl_name
or db_name.tbl_name
prefix for
a column reference in a statement unless the reference would be ambiguous.
For example, suppose tables t1
and t2
each contain a column
c
, and you retrieve c
in a SELECT
statement that uses
both t1
and t2
. In this case, c
is ambiguous because it
is not unique among the tables used in the statement, so you must indicate
which table you mean by writing t1.c
or t2.c
. Similarly, if
you are retrieving from a table t
in database db1
and from a
table t
in database db2
, you must refer to columns in those
tables as db1.t.col_name
and db2.t.col_name
.
The syntax .tbl_name
means the table tbl_name
in the current
database. This syntax is accepted for ODBC compatibility, because some ODBC
programs prefix table names with a `.' character.
In MySQL, databases and tables correspond to directories and files within those directories. Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are case sensitive in Unix and case insensitive in Win32.
Note: Although database and table names are case insensitive for
Win32, you should not refer to a given database or table using different
cases within the same query. The following query would not work because it
refers to a table both as my_table
and as MY_TABLE
:
mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;
Column names are case insensitive in all cases.
Aliases on tables are case sensitive. The following query would not work
because it refers to the alias both as a
and as A
:
mysql> SELECT col_name FROM tbl_name AS a WHERE a.col_name = 1 OR A.col_name = 2;
Aliases on columns are case insensitive.
MySQL supports thread specific variables with the
@variablename
syntax. A variable name may consist of
alphanumeric characters from the current character set and also
`_', `$', and `.' . The default character set is
ISO-8859-1 Latin1; this may be changed by recompiling
MySQL. See section 9.1.1 The character set used for data and sorting.
Variables don't have to be initialized. They contain NULL by default and can store and integer, real or a string value. All variables for a thread are automaticly freed when the thread exits.
You can set a variable with the SET
syntax:
SET @variable= { integer expression | real expression | string expression } [,@variable= ...].
You can also set a variable in an expression with the @variable:=expr
syntax:
select @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3; +----------------------+------+------+------+ | @t1:=(@t2:=1)+@t3:=4 | @t1 | @t2 | @t3 | +----------------------+------+------+------+ | 5 | 5 | 1 | 4 | +----------------------+------+------+------+
(We had to use the :=
syntax here as =
was reserverd for
comparisons)
MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (character) types. This section first gives an overview of the types available and summarizes the storage requirements for each column type, then provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.
The column types supported by MySQL are listed below. The following code letters are used in the descriptions:
M
D
M
-2.
Square brackets (`[' and `]') indicate parts of type specifiers that are optional.
Note that if you specify ZEROFILL
for a column, MySQL will
automatically add the UNSIGNED
attribute to the column.
TINYINT[(M)] [UNSIGNED] [ZEROFILL]
-128
to 127
. The
unsigned range is 0
to 255
.
SMALLINT[(M)] [UNSIGNED] [ZEROFILL]
-32768
to 32767
. The
unsigned range is 0
to 65535
.
MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]
-8388608
to
8388607
. The unsigned range is 0
to 16777215
.
INT[(M)] [UNSIGNED] [ZEROFILL]
-2147483648
to
2147483647
. The unsigned range is 0
to 4294967295
.
INTEGER[(M)] [UNSIGNED] [ZEROFILL]
INT
.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]
-9223372036854775808
to
9223372036854775807
. The unsigned range is 0
to
18446744073709551615
. Note that all arithmetic is done using
signed BIGINT
or DOUBLE
values, so you shouldn't use
unsigned big integers larger than 9223372036854775807
(63 bits)
except with bit functions! Note that -
, +
and *
will use BIGINT
arithmetic when both arguments are INTEGER
values! This means that if you multiply two big integers (or results
from functions that return integers) you may get unexpected results if
the result is larger than 9223372036854775807
.
A floating-point number. Cannot be unsigned. precision
can be
<=24
for a single precision floating point number and between 25
and 53 for a double precision floating point number.
these types are like the FLOAT
and DOUBLE
types described
immediately below. FLOAT(X)
have the same ranges as the
corresponding FLOAT
and DOUBLE
types, but the display
size and number of decimals is undefined.
In MySQL 3.23, this is a true floating point value. In
earlier MySQL versions, FLOAT(precision)
always has 2 decimals.
This syntax is provided for ODBC compatibility.
FLOAT[(M,D)] [ZEROFILL]
-3.402823466E+38
to
-1.175494351E-38
, 0
and 1.175494351E-38
to
3.402823466E+38
. The M is the display width and D is the
number of decimals. FLOAT
without an argument or with an argument of
<= 24 stands for a single-precision floating point number.
DOUBLE[(M,D)] [ZEROFILL]
-1.7976931348623157E+308
to
-2.2250738585072014E-308
, 0
and
2.2250738585072014E-308
to 1.7976931348623157E+308
. The M
is the display width and D is the number of decimals. DOUBLE
without an argument or FLOAT(X)
where 25 <= X <= 53 stands for a
double-precision floating point number.
DOUBLE PRECISION[(M,D)] [ZEROFILL]
REAL[(M,D)] [ZEROFILL]
DOUBLE
.
DECIMAL[(M[,D])] [ZEROFILL]
CHAR
column: ``unpacked'' means the number is stored as a string,
using one character for each digit of the value. The decimal point,
and, for negative numbers, the `-' sign is not counted in M. If
D
is 0, values will have no decimal point or fractional part.
The maximum range of DECIMAL
values is the same as for
DOUBLE
, but the actual range for a given DECIMAL
column
may be constrained by the choice of M
and D
.
If D
is left out it's set to 0. If M
is left out it's set to 10.
Note that in MySQL 3.22 the M
argument includes the sign
and the decimal point.
NUMERIC(M,D) [ZEROFILL]
DECIMAL
.
DATE
'1000-01-01'
to '9999-12-31'
.
MySQL displays DATE
values in 'YYYY-MM-DD'
format, but
allows you to assign values to DATE
columns using either strings or
numbers.
DATETIME
'1000-01-01
00:00:00'
to '9999-12-31 23:59:59'
. MySQL displays
DATETIME
values in 'YYYY-MM-DD HH:MM:SS'
format, but allows you
to assign values to DATETIME
columns using either strings or numbers.
TIMESTAMP[(M)]
'1970-01-01 00:00:00'
to sometime in the
year 2037
. MySQL displays TIMESTAMP
values in
YYYYMMDDHHMMSS
, YYMMDDHHMMSS
, YYYYMMDD
or YYMMDD
format, depending on whether M
is 14
(or missing), 12
,
8
or 6
, but allows you to assign values to TIMESTAMP
columns using either strings or numbers. A TIMESTAMP
column is useful
for recording the date and time of an INSERT
or UPDATE
operation because it is automatically set to the date and time of the most
recent operation if you don't give it a value yourself. You can also set it
to the current date and time by assigning it a NULL
value. See section 7.3.6 Date and time types.
TIME
'-838:59:59'
to '838:59:59'
.
MySQL displays TIME
values in 'HH:MM:SS'
format, but
allows you to assign values to TIME
columns using either strings or
numbers.
YEAR[(2|4)]
1901
to 2155
, and 0000
in the 4 year format and
1970-2069 if you use the 2 digit format (70-69). MySQL displays
YEAR
values in YYYY
format, but allows you to assign
values to YEAR
columns using either strings or numbers.
(The YEAR
type is new in MySQL 3.22.)
CHAR(M) [BINARY]
M
is 1 to 255 characters.
Trailing spaces are removed when the value is retrieved. CHAR
values
are sorted and compared in case-insensitive fashion according to the
default character set unless the BINARY
keyword is given.
NATIONAL CHAR
(short form NCHAR
) is the ANSI SQL way to
define that a CHAR column should use the default CHARACTER set. This is
default in MySQL
.
CHAR
is a shorthand for CHARACTER
.
[NATIONAL] VARCHAR(M) [BINARY]
M
is 1 to 255 characters.
VARCHAR
values are sorted and compared in case-insensitive fashion
unless the BINARY
keyword is given. See section 7.7.1 Silent column specification changes.
VARCHAR
is a shorthand for CHARACTER VARYING
.
TINYBLOB
TINYTEXT
BLOB
or TEXT
column with a maximum length of 255 (2^8 - 1)
characters. See section 7.7.1 Silent column specification changes.
BLOB
TEXT
BLOB
or TEXT
column with a maximum length of 65535 (2^16 - 1)
characters. See section 7.7.1 Silent column specification changes.
MEDIUMBLOB
MEDIUMTEXT
BLOB
or TEXT
column with a maximum length of 16777215
(2^24 - 1) characters. See section 7.7.1 Silent column specification changes.
LONGBLOB
LONGTEXT
BLOB
or TEXT
column with a maximum length of 4294967295
(2^32 - 1) characters. See section 7.7.1 Silent column specification changes.
ENUM('value1','value2',...)
'value1'
, 'value2'
, ...
, or NULL
. An ENUM
can have a maximum of 65535 distinct values.
SET('value1','value2',...)
'value1'
, 'value2'
,
...
A SET
can have a maximum of 64 members.
The storage requirements for each of the column types supported by MySQL are listed below by category.
Column type | Storage required |
TINYINT | 1 byte |
SMALLINT | 2 bytes |
MEDIUMINT | 3 bytes |
INT | 4 bytes |
INTEGER | 4 bytes |
BIGINT | 8 bytes |
FLOAT(X) | 4 if X <= 24 or 8 if 25 <= X <= 53 |
FLOAT | 4 bytes |
DOUBLE | 8 bytes |
DOUBLE PRECISION | 8 bytes |
REAL | 8 bytes |
DECIMAL(M,D) | M bytes (D +2, if M < D )
|
NUMERIC(M,D) | M bytes (D +2, if M < D )
|
Column type | Storage required |
DATE | 3 bytes |
DATETIME | 8 bytes |
TIMESTAMP | 4 bytes |
TIME | 3 bytes |
YEAR | 1 byte |
Column type | Storage required |
CHAR(M) | M bytes, 1 <= M <= 255
|
VARCHAR(M) | L +1 bytes, where L <= M and
1 <= M <= 255
|
TINYBLOB , TINYTEXT | L +1 bytes,
where L < 2^8
|
BLOB , TEXT | L +2 bytes,
where L < 2^16
|
MEDIUMBLOB , MEDIUMTEXT | L +3 bytes,
where L < 2^24
|
LONGBLOB , LONGTEXT | L +4 bytes,
where L < 2^32
|
ENUM('value1','value2',...) | 1 or 2 bytes, depending on the number of enumeration values (65535 values maximum) |
SET('value1','value2',...) | 1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum) |
VARCHAR
and the BLOB
and TEXT
types are variable-length
types, for which the storage requirements depend on the actual length of
column values (represented by L
in the preceding table), rather than
on the type's maximum possible size. For example, a VARCHAR(10)
column can hold a string with a maximum length of 10 characters. The actual
storage required is the length of the string (L
), plus 1 byte to
record the length of the string. For the string 'abcd'
, L
is 4
and the storage requirement is 5 bytes.
The BLOB
and TEXT
types require 1, 2, 3 or 4 bytes to record
the length of the column value, depending on the maximum possible length of
the type.
If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may under certain conditions change a column from a variable-length type to a fixed-length type, or vice-versa. See section 7.7.1 Silent column specification changes.
The size of an ENUM
object is determined by the number of different
enumeration values. 1 byte is used for enumerations with up to 255 possible
values. 2 bytes are used for enumerations with up to 65535 values.
The size of a SET
object is determined by the number of different
set members. If the set size is N
, the object occupies (N+7)/8
bytes, rounded up to 1, 2, 3, 4 or 8 bytes. A SET
can have a maximum
of 64 members.
MySQL supports all of the ANSI/ISO SQL92 numeric types. These
types include the exact numeric data types (NUMERIC
,
DECIMAL
, INTEGER
, and SMALLINT
), as well as the
approximate numeric data types (FLOAT
, REAL
, and
DOUBLE PRECISION
). The keyword INT
is a synonym for
INTEGER
, and the keyword DEC
is a synonym for
DECIMAL
.
The NUMERIC
and DECIMAL
types are implemented as the same
type by MySQL, as permitted by the SQL92 standard. They are
used for values for which it is important to preserve exact precision,
for example with monetary data. When declaring a column of one of these
types the precision and scale can be (and usually is) specified; for
example:
salary DECIMAL(9,2)
In this example, 9
(precision
) represents the number of
significant decimal digits which will be stored for values, and
2
(scale
) represents the number of digits which will be
stored following the decimal point. In this case, therefore, the range
of values which can be stored in the salary
column is from
-9999999.99
to 9999999.99
. In ANSI/ISO SQL92, the syntax
DECIMAL(p)
is equivalent to DECIMAL(p,0)
. Similarly, the
syntax DECIMAL
is equivalent to DECIMAL(p,0)
, where the
implementation is allowed to decide the value of p
.
MySQL does not currently support either of these variant forms
of the DECIMAL
/NUMERIC
data types. This is not generally
a serious problem, as the principal benefits of these types derive from
the ability to control both precision and scale explicitly.
DECIMAL
and NUMERIC
values are stored as strings, rather
than as binary floating point numbers, in order to preserve the decimal
precision of those values. One character is used for each digit of the
value, the decimal point (if scale
> 0) and the `-' sign
(for negative numbers). If scale
is 0, DECIMAL
and
NUMERIC
values contain no decimal point or fractional part.
The maximum range of DECIMAL
and NUMERIC
values is the
same as for DOUBLE
, but the actual range for a given
DECIMAL
or NUMERIC
column can be constrained by the
precision
or scale
for a given column. When such a column
is assigned a value with more digits following the decimal point than
are allowed by the specified scale
, the value is rounded to that
scale
. When a DECIMAL
or NUMERIC
column is
assigned a value whose magnitude exceeds the range implied by the
specified (or defaulted) precision
and scale
,
MySQL stores the value representing the corresponding end
point of that range.
As an extension to the ANSI/ISO SQL92 standard, MySQL also
supports the integral types TINYINT
, MEDIUMINT
, and
BIGINT
as listed in the tables above. Another extension is
supported by MySQL for optionally specifying the display width
of an integral value in parentheses following the base keyword for the
type (for example, INT(4)
). This optional width specification is
used to left-pad the display of values whose width is less than the
width specified for the column, but does not constrain the range of
values which can be stored in the column, nor the number of digits which
will be displayed for values whose width exceeds that specified for the
column. When used in conjunction with the optional extension attribute
ZEROFILL
the default padding of spaces is replaced with zeroes.
For example, for a column declared as INT(5) ZEROFILL
, a value
of 4
is retrieved as 00004
. Note that if you store larger
values than the display width in an integral column, you may experience
problems when MySQL generates temporary tables for some
complicated joins as in these case MySQL trust that the data
did fit into the original column width.
All integral types can have an optional (non-standard) attribute
UNSIGNED
. Unsigned values can be used when you want to allow
only positive numbers in a column and you need a little bigger numeric
range for the column.
The FLOAT
type is used to represent approximate numeric data
types. The ANSI/ISO SQL92 standard allows an optional specification of
the precision (but not the range of the exponent) in bits following the
keyword FLOAT
in parentheses. The MySQL implementation
also supports this optional precision specification. When the keyword
FLOAT
is used for a column type without a precision
specification, MySQL uses four bytes to store the values. A
variant syntax is also supported, with two numbers given in parentheses
following the FLOAT
keyword. With this option, the first number
continues to represent the storage requirements for the value in bytes,
and the second number specifies the number of digits to be stored and
displayed following the decimal point (as with DECIMAL
and
NUMERIC
). When MySQL is asked to store a number for
such a column with more decimal digits following the decimal point than
specified for the column, the value is rounded to eliminate the extra
digits when the value is stored.
The REAL
and DOUBLE PRECISION
types do not accept
precision specifications. As an extension to the ANSI/ISO SQL92
standard, MySQL recognizes DOUBLE
as a synonym for
the DOUBLE PRECISION
type. In contrast with the standard's
requirement that the precision for REAL
be smaller than
that used for DOUBLE PRECISION
, MySQL implements
both as 8-byte double-precision floating point values. For
maximum portability, code requiring storage of approximate numeric
data values should use FLOAT
or DOUBLE PRECISION
with
no specification of precision or number of decimal points.
When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.
For example, the range of an INT
column is -2147483648
to
2147483647
. If you try to insert -9999999999
into an
INT
column, the value is clipped to the lower endpoint of the range,
and -2147483648
is stored instead. Similarly, if you try to insert
9999999999
, 2147483647
is stored instead.
If the INT
column is UNSIGNED
, the size of the column's
range is the same but its endpoints shift up to 0
and 4294967295
.
If you try to store -9999999999
and 9999999999
,
the values stored in the column become 0
and 4294967296
.
Conversions that occur due to clipping are reported as ``warnings'' for
ALTER TABLE
, LOAD DATA INFILE
, UPDATE
and
multi-row INSERT
statements.
The date and time types are DATETIME
, DATE
, TIMESTAMP
,
TIME
and YEAR
. Each of these has a range of legal values, as
well as a ``zero'' value that is used when you specify an illegal value.
Here are some general considerations to keep in mind when working with date and time types:
'98-09-04'
), rather than in the
month-day-year or day-month-year orders commonly used elsewhere (e.g.,
'09-04-98'
, '04-09-98'
).
TIME
values
are clipped to the appropriate endpoint of the TIME
range.) The table
below shows the format of the ``zero'' value for each type:
Column type | ``Zero'' value |
DATETIME | '0000-00-00 00:00:00'
|
DATE | '0000-00-00'
|
TIMESTAMP | 00000000000000 (length depends on display size)
|
TIME | '00:00:00'
|
YEAR | 0000
|
'0'
or 0
, which are easier to write.
NULL
in MyODBC 2.50.12 and above, because
ODBC can't handle such values.
MySQL itself is Y2K-safe (see section 1.6 Year 2000 compliance), but input values presented to MySQL may not be. Any input containing 2-digit year values is ambiguous, since the century is unknown. Such values must be interpreted into 4-digit form since MySQL stores years internally using four digits.
For DATETIME
, DATE
, TIMESTAMP
and YEAR
types,
MySQL interprets dates with ambiguous year values using the
following rules:
00-69
are converted to 2000-2069
.
70-99
are converted to 1970-1999
.
Remember that these rules provide only reasonable guesses as to what your data mean. If the heuristics used by MySQL don't produce the correct values, you should provide unambiguous input containing 4-digit year values.
DATETIME
, DATE
and TIMESTAMP
types
The DATETIME
, DATE
and TIMESTAMP
types are related.
This section describes their characteristics, how they are similar and how
they differ.
The DATETIME
type is used when you need values that contain both date
and time information. MySQL retrieves and displays DATETIME
values in 'YYYY-MM-DD HH:MM:SS'
format. The supported range is
'1000-01-01 00:00:00'
to '9999-12-31 23:59:59'
. (``Supported''
means that although earlier values might work, there is no guarantee that
they will.)
The DATE
type is used when you need only a date value, without a time
part. MySQL retrieves and displays DATE
values in
'YYYY-MM-DD'
format. The supported range is '1000-01-01'
to
'9999-12-31'
.
The TIMESTAMP
column type provides a type that you can use to
automatically mark INSERT
or UPDATE
operations with the current
date and time. If you have multiple TIMESTAMP
columns, only the first
one is updated automatically.
Automatic updating of the first TIMESTAMP
column occurs under any of
the following conditions:
INSERT
or
LOAD DATA INFILE
statement.
UPDATE
statement and some
other column changes value. (Note that an UPDATE
that sets a column
to the value it already has will not cause the TIMESTAMP
column to be
updated, because if you set a column to its current value, MySQL
ignores the update for efficiency.)
TIMESTAMP
column to NULL
.
TIMESTAMP
columns other than the first may also be set to the current
date and time. Just set the column to NULL
, or to NOW()
.
You can set any TIMESTAMP
column to a value different than the current
date and time by setting it explicitly to the desired value. This is true
even for the first TIMESTAMP
column. You can use this property if,
for example, you want a TIMESTAMP
to be set to the current date and
time when you create a row, but not to be changed whenever the row is updated
later:
TIMESTAMP
column explicitly to its current value.
On the other hand, you may find it just as easy to use a DATETIME
column that you initialize to NOW()
when the row is created and
leave alone for subsequent updates.
TIMESTAMP
values may range from the beginning of 1970 to sometime in
the year 2037, with a resolution of one second. Values are displayed as
numbers.
The format in which MySQL retrieves and displays TIMESTAMP
values depends on the display size, as illustrated by the table below. The
`full' TIMESTAMP
format is 14 digits, but TIMESTAMP
columns may
be created with shorter display sizes:
Column type | Display format |
TIMESTAMP(14) | YYYYMMDDHHMMSS
|
TIMESTAMP(12) | YYMMDDHHMMSS
|
TIMESTAMP(10) | YYMMDDHHMM
|
TIMESTAMP(8) | YYYYMMDD
|
TIMESTAMP(6) | YYMMDD
|
TIMESTAMP(4) | YYMM
|
TIMESTAMP(2) | YY
|
All TIMESTAMP
columns have the same storage size, regardless of
display size. The most common display sizes are 6, 8, 12, and 14. You can
specify an arbitrary display size at table creation time, but values of 0 or
greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to
13 are coerced to the next higher even number.
You can specify DATETIME
, DATE
and TIMESTAMP
values using
any of a common set of formats:
'YYYY-MM-DD HH:MM:SS'
or 'YY-MM-DD
HH:MM:SS'
format. A ``relaxed'' syntax is allowed--any punctuation
character may be used as the delimiter between date parts or time parts.
For example, '98-12-31 11:30:45'
, '98.12.31 11+30+45'
,
'98/12/31 11*30*45'
and '98@12@31 11^30^45'
are
equivalent.
'YYYY-MM-DD'
or 'YY-MM-DD'
format.
A ``relaxed'' syntax is allowed here, too. For example, '98-12-31'
,
'98.12.31'
, '98/12/31'
and '98@12@31'
are
equivalent.
'YYYYMMDDHHMMSS'
or
'YYMMDDHHMMSS'
format, provided that the string makes sense as a
date. For example, '19970523091528'
and '970523091528'
are
interpreted as '1997-05-23 09:15:28'
, but '971122459015'
is
illegal (it has a nonsensical minute part) and becomes '0000-00-00
00:00:00'
.
'YYYYMMDD'
or 'YYMMDD'
format, provided that the string makes sense as a date. For example,
'19970523'
and '970523'
are interpreted as
'1997-05-23'
, but '971332'
is illegal (it has nonsensical month
and day parts) and becomes '0000-00-00'
.
YYYYMMDDHHMMSS
or YYMMDDHHMMSS
format, provided that the number makes sense as a date. For example,
19830905132800
and 830905132800
are interpreted as
'1983-09-05 13:28:00'
.
YYYYMMDD
or YYMMDD
format, provided that the number makes sense as a date. For example,
19830905
and 830905
are interpreted as '1983-09-05'
.
DATETIME
, DATE
or TIMESTAMP
context, such as
NOW()
or CURRENT_DATE
.
Illegal DATETIME
, DATE
or TIMESTAMP
values are converted
to the ``zero'' value of the appropriate type ('0000-00-00 00:00:00'
,
'0000-00-00'
or 00000000000000
).
For values specified as strings that include date part delimiters, it is not
necessary to specify two digits for month or day values that are less than
10
. '1979-6-9'
is the same as '1979-06-09'
. Similarly,
for values specified as strings that include time part delimiters, it is not
necessary to specify two digits for hour, month or second values that are
less than 10
. '1979-10-30 1:2:3'
is the same as
'1979-10-30 01:02:03'
.
Values specified as numbers should be 6, 8, 12 or 14 digits long. If the
number is 8 or 14 digits long, it is assumed to be in YYYYMMDD
or
YYYYMMDDHHMMSS
format and that the year is given by the first 4
digits. If the number is 6 or 12 digits long, it is assumed to be in
YYMMDD
or YYMMDDHHMMSS
format and that the year is given by the
first 2 digits. Numbers that are not one of these lengths are interpreted
as though padded with leading zeros to the closest length.
Values specified as non-delimited strings are interpreted using their length
as given. If the string is 8 or 14 characters long, the year is assumed to
be given by the first 4 characters. Otherwise the year is assumed to be
given by the first 2 characters. The string is interpreted from left to
right to find year, month, day, hour, minute and second values, for as many
parts as are present in the string. This means you should not use strings
that have fewer than 6 characters. For example, if you specify '9903'
,
thinking that will represent March, 1999, you will find that MySQL
inserts a ``zero'' date into your table. This is because the year and month
values are 99
and 03
, but the day part is missing (zero), so
the value is not a legal date.
TIMESTAMP
columns store legal values using the full precision with
which the value was specified, regardless of the display size. This has
several implications:
TIMESTAMP(4)
or TIMESTAMP(2)
. Otherwise, the value will not
be a legal date and 0
will be stored.
ALTER TABLE
to widen a narrow TIMESTAMP
column,
information will be displayed that previously was ``hidden''.
TIMESTAMP
column does not cause information to
be lost, except in the sense that less information is shown when the values
are displayed.
TIMESTAMP
values are stored to full precision, the only
function that operates directly on the underlying stored value is
UNIX_TIMESTAMP()
. Other functions operate on the formatted retrieved
value. This means you cannot use functions such as HOUR()
or
SECOND()
unless the relevant part of the TIMESTAMP
value is
included in the formatted value. For example, the HH
part of a
TIMESTAMP
column is not displayed unless the display size is at least
10, so trying to use HOUR()
on shorter TIMESTAMP
values
produces a meaningless result.
You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:
DATE
value to a DATETIME
or TIMESTAMP
object, the time part of the resulting value is set to '00:00:00'
,
because the DATE
value contains no time information.
DATETIME
or TIMESTAMP
value to a DATE
object, the time part of the resulting value is deleted, because the
DATE
type stores no time information.
DATETIME
, DATE
and TIMESTAMP
values all can be specified using the same set of formats, the types do not
all have the same range of values. For example, TIMESTAMP
values
cannot be earlier than 1970
or later than 2037
. This means
that a date such as '1968-01-01'
, while legal as a DATETIME
or
DATE
value, is not a valid TIMESTAMP
value and will be
converted to 0
if assigned to such an object.
Be aware of certain pitfalls when specifying date values:
'10:11:12'
might look like a time value
because of the `:' delimiter, but if used in a date context will be
interpreted as the year '2010-11-12'
. The value '10:45:15'
will be converted to '0000-00-00'
because '45'
is not a legal
month.
00-69
are converted to 2000-2069
.
70-99
are converted to 1970-1999
.
TIME
type
MySQL retrieves and displays TIME
values in 'HH:MM:SS'
format (or 'HHH:MM:SS'
format for large hours values). TIME
values may range from '-838:59:59'
to '838:59:59'
. The reason
the hours part may be so large is that the TIME
type may be used not
only to represent a time of day (which must be less than 24 hours), but also
elapsed time or a time interval between two events (which may be much greater
than 24 hours, or even negative).
You can specify TIME
values in a variety of formats:
'HH:MM:SS'
format.
A ``relaxed'' syntax is allowed--any punctuation character may be
used as the delimiter between time parts. For example, '10:11:12'
and '10.11.12'
are equivalent.
'HHMMSS'
format, provided that
it makes sense as a time. For example, '101112'
is understood as
'10:11:12'
, but '109712'
is illegal (it has a nonsensical
minute part) and becomes '00:00:00'
.
HHMMSS
format, provided that it makes sense as a time.
For example, 101112
is understood as '10:11:12'
.
TIME
context, such as CURRENT_TIME
.
For TIME
values specified as strings that include a time part
delimiter, it is not necessary to specify two digits for hours, minutes or
seconds values that are less than 10
. '8:3:2'
is the same as
'08:03:02'
.
Be careful about assigning ``short'' TIME
values to a TIME
column. MySQL interprets values using the assumption that the
rightmost digits represent seconds. (MySQL interprets TIME
values as elapsed time, rather than as time of day.) For example, you might
think of '11:12'
, '1112'
and 1112
as meaning
'11:12:00'
(12 minutes after 11 o'clock), but MySQL
interprets them as '00:11:12'
(11 minutes, 12 seconds). Similarly,
'12'
and 12
are interpreted as '00:00:12'
.
Values that lie outside the TIME
range
but are otherwise legal are clipped to the appropriate
endpoint of the range. For example, '-850:00:00'
and
'850:00:00'
are converted to '-838:59:59'
and
'838:59:59'
.
Illegal TIME
values are converted to '00:00:00'
. Note that
since '00:00:00'
is itself a legal TIME
value, there is no way
to tell, from a value of '00:00:00'
stored in a table, whether the
original value was specified as '00:00:00'
or whether it was illegal.
YEAR
type
The YEAR
type is a 1-byte type used for representing years.
MySQL retrieves and displays YEAR
values in YYYY
format. The range is 1901
to 2155
.
You can specify YEAR
values in a variety of formats:
'1901'
to '2155'
.
1901
to 2155
.
'00'
to '99'
. Values in the
ranges '00'
to '69'
and '70'
to '99'
are
converted to YEAR
values in the ranges 2000
to 2069
and
1970
to 1999
.
1
to 99
. Values in the
ranges 1
to 69
and 70
to 99
are converted to
YEAR
values in the ranges 2001
to 2069
and 1970
to 1999
. Note that the range for two-digit numbers is slightly
different than the range for two-digit strings, since you cannot specify zero
directly as a number and have it be interpreted as 2000
. You
must specify it as a string '0'
or '00'
or it will be
interpreted as 0000
.
YEAR
context, such as NOW()
.
Illegal YEAR
values are converted to 0000
.
The string types are CHAR
, VARCHAR
, BLOB
, TEXT
,
ENUM
and SET
.
CHAR
and VARCHAR
types
The CHAR
and VARCHAR
types are similar, but differ in the
way they are stored and retrieved.
The length of a CHAR
column is fixed to the length that you declare
when you create the table. The length can be any value between 1 and 255.
(As of MySQL 3.23, the length of CHAR
may be 0 to 255.)
When CHAR
values are stored, they are right-padded with spaces to the
specified length. When CHAR
values are retrieved, trailing spaces are
removed.
Values in VARCHAR
columns are variable-length strings. You can
declare a VARCHAR
column to be any length between 1 and 255, just as
for CHAR
columns. However, in contrast to CHAR
, VARCHAR
values are stored using only as many characters as are needed, plus one byte
to record the length. Values are not padded; instead, trailing spaces are
removed when values are stored. (This space removal differs from the ANSI
SQL specification.)
If you assign a value to a CHAR
or VARCHAR
column that
exceeds the column's maximum length, the value is truncated to fit.
The table below illustrates the differences between the two types of columns
by showing the result of storing various string values into CHAR(4)
and VARCHAR(4)
columns:
Value | CHAR(4) | Storage required | VARCHAR(4) | Storage required |
'' | ' ' | 4 bytes | '' | 1 byte |
'ab' | 'ab ' | 4 bytes | 'ab' | 3 bytes |
'abcd' | 'abcd' | 4 bytes | 'abcd' | 5 bytes |
'abcdefgh' | 'abcd' | 4 bytes | 'abcd' | 5 bytes |
The values retrieved from the CHAR(4)
and VARCHAR(4)
columns
will be the same in each case, because trailing spaces are removed from
CHAR
columns upon retrieval.
Values in CHAR
and VARCHAR
columns are sorted and compared in
case-insensitive fashion, unless the BINARY
attribute was specified
when the table was created. The BINARY
attribute means that column
values are sorted and compared in case-sensitive fashion according to the
ASCII order of the machine where the MySQL server is running.
The BINARY
attribute is ``sticky''. This means that if a column marked
BINARY
is used in an expression, the whole expression is compared as a
BINARY
value.
MySQL may silently change the type of a CHAR
or VARCHAR
column at table creation time.
See section 7.7.1 Silent column specification changes.
BLOB
and TEXT
types
A BLOB
is a binary large object that can hold a variable amount of
data. The four BLOB
types TINYBLOB
, BLOB
,
MEDIUMBLOB
and LONGBLOB
differ only in the maximum length of
the values they can hold.
See section 7.3.1 Column type storage requirements.
The four TEXT
types TINYTEXT
, TEXT
, MEDIUMTEXT
and LONGTEXT
correspond to the four BLOB
types and have the
same maximum lengths and storage requirements. The only difference between
BLOB
and TEXT
types is that sorting and comparison is performed
in case-sensitive fashion for BLOB
values and case-insensitive fashion
for TEXT
values. In other words, a TEXT
is a case-insensitive
BLOB
.
If you assign a value to a BLOB
or TEXT
column that exceeds
the column type's maximum length, the value is truncated to fit.
In most respects, you can regard a TEXT
column as a VARCHAR
column that can be as big as you like. Similarly, you can regard a
BLOB
column as a VARCHAR BINARY
column. The differences are:
BLOB
and TEXT
columns with
MySQL versions 3.23.2 and newer. Older versions of
MySQL did not support this.
BLOB
and TEXT
columns
when values are stored, as there is for VARCHAR
columns.
BLOB
and TEXT
columns cannot have DEFAULT
values.
MyODBC defines BLOB
values as LONGVARBINARY
and
TEXT
values as LONGVARCHAR
.
Because BLOB
and TEXT
values may be extremely long, you
may run up against some constraints when using them:
GROUP BY
or ORDER BY
on a BLOB
or
TEXT
column, you must convert the column value into a fixed-length
object. The standard way to do this is with the SUBSTRING
function. For example:
mysql> select comment from tbl_name,substring(comment,20) as substr ORDER BY substr;If you don't do this, only the first
max_sort_length
bytes of the
column are used when sorting. The default value of max_sort_length
is
1024; this value can be changed using the -O
option when starting the
mysqld
server. You can group on an expression involving BLOB
or
TEXT
values by specifying the column position or by using an alias:
mysql> select id,substring(blob_col,1,100) from tbl_name GROUP BY 2; mysql> select id,substring(blob_col,1,100) as b from tbl_name GROUP BY b;
BLOB
or TEXT
object is determined by its
type, but the largest value you can actually transmit between the client and
server is determined by the amount of available memory and the size of the
communications buffers. You can change the message buffer size, but you must
do so on both the server and client ends. See section 10.2.3 Tuning server parameters.
Note that each BLOB
or TEXT
value is represented internally by
a separately-allocated object. This is in contrast to all other column types,
for which storage is allocated once per column when the table is opened.
ENUM
type
An ENUM
is a string object whose value normally is chosen from a list
of allowed values that are enumerated explicitly in the column specification
at table creation time.
The value may also be the empty string (""
) or NULL
under
certain circumstances:
ENUM
(that is, a string not
present in the list of allowed values), the empty string is inserted
instead as a special error value.
ENUM
is declared NULL
, NULL
is also a legal value
for the column, and the default value is NULL
. If an ENUM
is
declared NOT NULL
, the default value is the first element of the
list of allowed values.
Each enumeration value has an index:
SELECT
statement to find rows into which invalid
ENUM
values were assigned:
mysql> SELECT * FROM tbl_name WHERE enum_col=0;
NULL
value is NULL
.
For example, a column specified as ENUM("one", "two", "three")
can
have any of the values shown below. The index of each value is also shown:
Value | Index |
NULL | NULL
|
"" | 0 |
"one" | 1 |
"two" | 2 |
"three" | 3 |
An enumeration can have a maximum of 65535 elements.
Lettercase is irrelevant when you assign values to an ENUM
column.
However, values retrieved from the column later have lettercase matching the
values that were used to specify the allowable values at table creation time.
If you retrieve an ENUM
in a numeric context, the column value's index
is returned. If you store a number into an ENUM
, the number is
treated as an index, and the the value stored is the enumeration member with
that index.
ENUM
values are sorted according to the order in which the enumeration
members were listed in the column specification. (In other words,
ENUM
values are sorted according to their index numbers.) For
example, "a"
sorts before "b"
for ENUM("a", "b")
, but
"b"
sorts before "a"
for ENUM("b", "a")
. The empty
string sorts before non-empty strings, and NULL
values sort before
all other enumeration values.
If you want to get all possible values for an ENUM
column, you should
use: SHOW COLUMNS FROM table_name LIKE enum_column_name
and parse
the ENUM
definition in the second column.
SET
type
A SET
is a string object that can have zero or more values, each of
which must be chosen from a list of allowed values specified when the table
is created. SET
column values that consist of multiple set members
are specified with members separated by commas (`,'). A consequence of
this is that SET
member values cannot themselves contain commas.
For example, a column specified as SET("one", "two") NOT NULL
can have
any of these values:
"" "one" "two" "one,two"
A SET
can have a maximum of 64 different members.
MySQL stores SET
values numerically, with the low-order bit
of the stored value corresponding to the first set member. If you retrieve a
SET
value in a numeric context, the value retrieved has bits set
corresponding to the set members that make up the column value. If a number
is stored into a SET
column, the bits that are set in the binary
representation of the number determine the set members in the column value.
Suppose a column is specified as SET("a","b","c","d")
. Then the
members have the following bit values:
SET member | Decimal value | Binary value |
a | 1 | 0001
|
b | 2 | 0010
|
c | 4 | 0100
|
d | 8 | 1000
|
If you assign a value of 9
to this column, that is 1001
in
binary, so the first and fourth SET
value members "a"
and
"d"
are selected and the resulting value is "a,d"
.
For a value containing more than one SET
element, it does not matter
what order the elements are listed in when you insert the value. It also
doesn't not matter how many times a given element is listed in the value.
When the value is retrieved later, each element in the value will appear
once, with elements listed according to the order in which they were
specified at table creation time. For example, if a column is specified as
SET("a","b","c","d")
, then "a,d"
, "d,a"
and
"d,a,a,d,d"
will all appear as "a,d"
when retrieved.
SET
values are sorted numerically. NULL
values sort before
non-NULL
SET
values.
Normally, you perform a SELECT
on a SET
column using
the LIKE
operator or the FIND_IN_SET()
function:
mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%'; mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;
But the following will also work:
mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2'; mysql> SELECT * FROM tbl_name WHERE set_col & 1;
The first of these statements looks for an exact match. The second looks for values containing the first set member.
If you want to get all possible values for an SET
column, you should
use: SHOW COLUMNS FROM table_name LIKE set_column_name
and parse
the SET
definition in the second column.
For the most efficient use of storage, try to use the most precise type in
all cases. For example, if an integer column will be used for values in the
range between 1
and 99999
, MEDIUMINT UNSIGNED
is the
best type.
Accurate representation of monetary values is a common problem. In
MySQL, you should use the DECIMAL
type. This is stored as
a string, so no loss of accuracy should occur. If accuracy is not
too important, the DOUBLE
type may also be good enough.
For high precision, you can always convert to a fixed-point type stored
in a BIGINT
. This allows you to do all calculations with integers
and convert results back to floating-point values only when necessary.
See section 10.6 Choosing a table type.
All MySQL column types can be indexed. Use of indexes on the
relevant columns is the best way to improve the performance of SELECT
operations.
A table may have up to 16 indexes. The maximum index length is 256 bytes, although this may be changed when compiling MySQL.
For CHAR
and VARCHAR
columns, you can index a prefix of a
column. This is much faster and requires less disk space than indexing the
whole column. The syntax to use in the CREATE TABLE
statement to
index a column prefix looks like this:
KEY index_name (col_name(length))
The example below creates an index for the first 10 characters of the
name
column:
mysql> CREATE TABLE test ( name CHAR(200) NOT NULL, KEY index_name (name(10)));
For BLOB
and TEXT
columns, you must index a prefix of the
column, you cannot index the entire thing.
MySQL can create indexes on multiple columns. An index may
consist of up to 15 columns. (On CHAR
and VARCHAR
columns you
can also use a prefix of the column as a part of an index).
A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.
MySQL uses multiple-column indexes in such a way that queries are
fast when you specify a known quantity for the first column of the index in a
WHERE
clause, even if you don't specify values for the other columns.
Suppose a table is created using the following specification:
mysql> CREATE TABLE test ( id INT NOT NULL, last_name CHAR(30) NOT NULL, first_name CHAR(30) NOT NULL, PRIMARY KEY (id), INDEX name (last_name,first_name));
Then the index name
is an index over last_name
and
first_name
. The index will be used for queries that specify
values in a known range for last_name
, or for both last_name
and first_name
.
Therefore, the name
index will be used in the following queries:
mysql> SELECT * FROM test WHERE last_name="Widenius"; mysql> SELECT * FROM test WHERE last_name="Widenius" AND first_name="Michael"; mysql> SELECT * FROM test WHERE last_name="Widenius" AND (first_name="Michael" OR first_name="Monty"); mysql> SELECT * FROM test WHERE last_name="Widenius" AND first_name >="M" AND first_name < "N";
However, the name
index will NOT be used in the following queries:
mysql> SELECT * FROM test WHERE first_name="Michael"; mysql> SELECT * FROM test WHERE last_name="Widenius" OR first_name="Michael";
For more information on the manner in which MySQL uses indexes to improve query performance, see section 10.4 MySQL index use.
To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the table below. These mappings make it easier to move table definitions from other database engines to MySQL:
Other vendor type | MySQL type |
BINARY(NUM) | CHAR(NUM) BINARY
|
CHAR VARYING(NUM) | VARCHAR(NUM)
|
FLOAT4 | FLOAT
|
FLOAT8 | DOUBLE
|
INT1 | TINYINT
|
INT2 | SMALLINT
|
INT3 | MEDIUMINT
|
INT4 | INT
|
INT8 | BIGINT
|
LONG VARBINARY | MEDIUMBLOB
|
LONG VARCHAR | MEDIUMTEXT
|
MIDDLEINT | MEDIUMINT
|
VARBINARY(NUM) | VARCHAR(NUM) BINARY
|
Column type mapping occurs at table creation time. If you create a table
with types used by other vendors and then issue a DESCRIBE tbl_name
statement, MySQL reports the table structure using the equivalent
MySQL types.
SELECT
and WHERE
clauses
A select_expression
or where_definition
in a SQL statement
can consist of any expression using the functions described below.
An expression that contains NULL
always produces a NULL
value
unless otherwise indicated in the documentation for the operators and
functions involved in the expression.
Note: There must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around arguments are permitted, though.
For the sake of brevity, examples display the output from the mysql
program in abbreviated form. So this:
mysql> select MOD(29,9); 1 rows in set (0.00 sec) +-----------+ | mod(29,9) | +-----------+ | 2 | +-----------+
Is displayed like this:
mysql> select MOD(29,9); -> 2
( ... )
mysql> select 1+2*3; -> 7 mysql> select (1+2)*3; -> 9
The usual arithmetic operators are available. Note that in the case of
-
, +
and *
, the result is calculated with BIGINT
(64-bit) precision if both arguments are integers!
+
mysql> select 3+5; -> 8
-
mysql> select 3-5; -> -2
*
mysql> select 3*5; -> 15 mysql> select 18014398509481984*18014398509481984.0; -> 324518553658426726783156020576256.0 mysql> select 18014398509481984*18014398509481984; -> 0The result of the last expression is incorrect because the result of the integer multiplication exceeds the 64-bit range of
BIGINT
calculations.
/
mysql> select 3/5; -> 0.60Division by zero produces a
NULL
result:
mysql> select 102/(1-1); -> NULLA division will be calculated with
BIGINT
arithmetic only if performed
in a context where its result is converted to an integer!
MySQL uses BIGINT
(64-bit) arithmetic for bit operations, so
these operators have a maximum range of 64 bits.
|
mysql> select 29 | 15; -> 31
&
mysql> select 29 & 15; -> 13
<<
BIGINT
) number to the left.
mysql> select 1 << 2 -> 4
>>
BIGINT
) number to the right.
mysql> select 4 >> 2 -> 1
~
mysql> select 5 & ~1 -> 4
BIT_COUNT(N)
N
.
mysql> select BIT_COUNT(29); -> 4
All logical functions return 1
(TRUE) or 0
(FALSE).
NOT
!
1
if the argument is 0
, otherwise returns
0
.
Exception: NOT NULL
returns NULL
.
mysql> select NOT 1; -> 0 mysql> select NOT NULL; -> NULL mysql> select ! (1+1); -> 0 mysql> select ! 1+1; -> 1The last example returns
1
because the expression evaluates
the same way as (!1)+1
.
OR
||
1
if either argument is not 0
and not
NULL
.
mysql> select 1 || 0; -> 1 mysql> select 0 || 0; -> 0 mysql> select 1 || NULL; -> 1
AND
&&
0
if either argument is 0
or NULL
,
otherwise returns 1
.
mysql> select 1 && NULL; -> 0 mysql> select 1 && 0; -> 0
Comparison operations result in a value of 1
(TRUE), 0
(FALSE)
or NULL
. These functions work for both numbers and strings. Strings
are automatically converted to numbers and numbers to strings as needed (as
in Perl).
MySQL performs comparisons using the following rules:
NULL
, the result of the comparison
is NULL
, except for the <=>
operator.
TIMESTAMP
or DATETIME
column and
the other argument is a constant, the constant is converted
to a timestamp before the comparison is performed. This is done to be more
ODBC-friendly.
By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).
The examples below illustrate conversion of strings to numbers for comparison operations:
mysql> SELECT 1 > '6x'; -> 0 mysql> SELECT 7 > '6x'; -> 1 mysql> SELECT 0 > 'x6'; -> 0 mysql> SELECT 0 = 'x6'; -> 1
=
mysql> select 1 = 0; -> 0 mysql> select '0' = 0; -> 1 mysql> select '0.0' = 0; -> 1 mysql> select '0.01' = 0; -> 0 mysql> select '.01' = 0.01; -> 1
<>
!=
mysql> select '.01' <> '0.01'; -> 1 mysql> select .01 <> '0.01'; -> 0 mysql> select 'zapp' <> 'zappp'; -> 1
<=
mysql> select 0.1 <= 2; -> 1
<
mysql> select 2 <= 2; -> 1
>=
mysql> select 2 >= 2; -> 1
>
mysql> select 2 > 2; -> 0
<=>
mysql> select 1 <=> 1, NULL <=> NULL, 1 <=> NULL; -> 1 1 0
IS NULL
IS NOT NULL
NULL
mysql> select 1 IS NULL, 0 IS NULL, NULL IS NULL: -> 0 0 1 mysql> select 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL; -> 1 1 0
expr BETWEEN min AND max
expr
is greater than or equal to min
and expr
is less
than or equal to max
, BETWEEN
returns 1
, otherwise it
returns 0
. This is equivalent to the expression (min
<= expr AND expr <= max)
if all the arguments are of the same type.
The first argument (expr
) determines how the comparison is performed.
If expr
is a case-insensitive string expression, a case-insensitive
string comparison is done. If expr
is a case-sensitive string
expression, a case-sensitive string comparison is done. If expr
is an integer expression, an integer comparison is done. Otherwise,
a floating-point (real) comparison is done.
mysql> select 1 BETWEEN 2 AND 3; -> 0 mysql> select 'b' BETWEEN 'a' AND 'c'; -> 1 mysql> select 2 BETWEEN 2 AND '3'; -> 1 mysql> select 2 BETWEEN 2 AND 'x-3'; -> 0
expr IN (value,...)
1
if expr
is any of the values in the IN
list,
else returns 0
. If all values are constants, then all values are
evaluated according to the type of expr
and sorted. The search for the
item is then done using a binary search. This means IN
is very quick
if the IN
value list consists entirely of constants. If expr
is a case-sensitive string expression, the string comparison is performed in
case-sensitive fashion.
mysql> select 2 IN (0,3,5,'wefwf'); -> 0 mysql> select 'wefwf' IN (0,3,5,'wefwf'); -> 1
expr NOT IN (value,...)
NOT (expr IN (value,...))
.
ISNULL(expr)
expr
is NULL
, ISNULL()
returns 1
, otherwise
it returns 0
.
mysql> select ISNULL(1+1); -> 0 mysql> select ISNULL(1/0); -> 1Note that a comparison of
NULL
values using =
will always be
false!
COALESCE(list)
NULL
element in list.
mysql> select COALESCE(NULL,1); -> 1 mysql> select COALESCE(NULL,NULL,NULL); -> NULL
INTERVAL(N,N1,N2,N3,...)
0
if N
< N1
, 1
if N
< N2
and so on. All arguments are treated as integers. It is required that
N1
< N2
< N3
< ...
< Nn
for this function
to work correctly. This is because a binary search is used (very fast).
mysql> select INTERVAL(23, 1, 15, 17, 30, 44, 200); -> 3 mysql> select INTERVAL(10, 1, 10, 100, 1000); -> 2 mysql> select INTERVAL(22, 23, 30, 44, 200); -> 0
Normally, if any expression in a string comparison is case sensitive, the comparison is performed in case-sensitive fashion.
expr LIKE pat [ESCAPE 'escape-char']
1
(TRUE) or 0
(FALSE). With LIKE
you can use the following two wildcard characters
in the pattern:
% | Matches any number of characters, even zero characters |
_ | Matches exactly one character |
mysql> select 'David!' LIKE 'David_'; -> 1 mysql> select 'David!' LIKE '%D%v%'; -> 1To test for literal instances of a wildcard character, precede the character with the escape character. If you don't specify the
ESCAPE
character,
`\' is assumed:
\% | Matches one % character
|
\_ | Matches one _ character
|
mysql> select 'David!' LIKE 'David\_'; -> 0 mysql> select 'David_' LIKE 'David\_'; -> 1To specify a different escape character, use the
ESCAPE
clause:
mysql> select 'David_' LIKE 'David|_' ESCAPE '|'; -> 1
LIKE
is allowed on numeric expressions! (This is a MySQL
extension to the ANSI SQL LIKE
.)
mysql> select 10 LIKE '1%'; -> 1Note: Because MySQL uses the C escape syntax in strings (e.g., `\n'), you must double any `\' that you use in your
LIKE
strings. For example, to search for `\n', specify it as `\\n'. To
search for `\', specify it as `\\\\' (the backslashes are stripped
once by the parser, and another time when the pattern match is done, leaving
a single backslash to be matched).
expr NOT LIKE pat [ESCAPE 'escape-char']
NOT (expr LIKE pat [ESCAPE 'escape-char'])
.
expr REGEXP pat
expr RLIKE pat
expr
against a pattern
pat
. The pattern can be an extended regular expression.
See section H Description of MySQL regular expression syntax. Returns 1
if expr
matches pat
, otherwise
returns 0
. RLIKE
is a synonym for REGEXP
, provided for
mSQL
compatibility. Note: Because MySQL uses the C escape
syntax in strings (e.g., `\n'), you must double any `\' that you
use in your REGEXP
strings. In MySQL
3.23.4
REGEXP
is case insensitive for normal (not binary) strings.
mysql> select 'Monty!' REGEXP 'm%y%%'; -> 0 mysql> select 'Monty!' REGEXP '.*'; -> 1 mysql> select 'new*\n*line' REGEXP 'new\\*.\\*line'; -> 1 mysql> select "a" REGEXP "A", "a" REGEXP BINARY "A"; -> 1 0
REGEXP
and RLIKE
use the current character set (ISO-8859-1
Latin1 by default) when deciding the type of a character.
expr NOT REGEXP pat
expr NOT RLIKE pat
NOT (expr REGEXP pat)
.
STRCMP(expr1,expr2)
STRCMP()
returns 0
if the strings are the same, -1
if the first
argument is smaller than the second according to the current sort order,
and 1
otherwise.
mysql> select STRCMP('text', 'text2'); -> -1 mysql> select STRCMP('text2', 'text'); -> 1 mysql> select STRCMP('text', 'text'); -> 0
BINARY
BINARY
operator casts the string following it to a binary string.
This is an easy way to force a column comparison to be case sensitive even
if the column isn't defined as BINARY
or BLOB
.
mysql> select "a" = "A"; -> 1 mysql> select BINARY "a" = "A"; -> 0
BINARY
was introduced in MySQL 3.23.0
IFNULL(expr1,expr2)
expr1
is not NULL
, IFNULL()
returns expr1
,
else it returns expr2
. IFNULL()
returns a numeric or string
value, depending on the context in which it is used.
mysql> select IFNULL(1,0); -> 1 mysql> select IFNULL(0,10); -> 0 mysql> select IFNULL(1/0,10); -> 10 mysql> select IFNULL(1/0,'yes'); -> 'yes'
IF(expr1,expr2,expr3)
expr1
is TRUE (expr1 <> 0
and expr1 <> NULL
) then
IF()
returns expr2
, else it returns expr3
.
IF()
returns a numeric or string value, depending on the context
in which it is used.
mysql> select IF(1>2,2,3); -> 3 mysql> select IF(1<2,'yes','no'); -> 'yes' mysql> select IF(strcmp('test','test1'),'yes','no'); -> 'no'
expr1
is evaluated as an integer value, which means that if you are
testing floating-point or string values, you should do so using a comparison
operation.
mysql> select IF(0.1,1,0); -> 0 mysql> select IF(0.1<>0,1,0); -> 1In the first case above,
IF(0.1)
returns 0
because 0.1
is converted to an integer value, resulting in a test of IF(0)
. This
may not be what you expect. In the second case, the comparison tests the
original floating-point value to see whether it is non-zero. The result
of the comparison is used as an integer.
CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END
result
where
value=compare-value
. The second version returns the result for
the first condition which is true. If there was no matching result
value, then the result after ELSE
is returned. If there is no
ELSE
part then NULL
is returned.
mysql> SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END; -> "one" mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END; -> "true" mysql> SELECT CASE BINARY "B" when "a" then 1 when "b" then 2 END; -> NULL
All mathematical functions return NULL
in case of an error.
-
mysql> select - 2; -> -2Note that if this operator is used with a
BIGINT
, the return value is a
BIGINT
! This means that you should avoid using -
on integers that
may have the value of -2^63
!
ABS(X)
X
.
mysql> select ABS(2); -> 2 mysql> select ABS(-32); -> 32This function is safe to use with
BIGINT
values.
SIGN(X)
-1
, 0
or 1
, depending
on whether X
is negative, zero, or positive.
mysql> select SIGN(-32); -> -1 mysql> select SIGN(0); -> 0 mysql> select SIGN(234); -> 1
MOD(N,M)
%
%
operator in C).
Returns the remainder of N
divided by M
.
mysql> select MOD(234, 10); -> 4 mysql> select 253 % 7; -> 1 mysql> select MOD(29,9); -> 2This function is safe to use with
BIGINT
values.
FLOOR(X)
X
.
mysql> select FLOOR(1.23); -> 1 mysql> select FLOOR(-1.23); -> -2Note that the return value is converted to a
BIGINT
!
CEILING(X)
X
.
mysql> select CEILING(1.23); -> 2 mysql> select CEILING(-1.23); -> -1Note that the return value is converted to a
BIGINT
!
ROUND(X)
X
, rounded to an integer.
mysql> select ROUND(-1.23); -> -1 mysql> select ROUND(-1.58); -> -2 mysql> select ROUND(1.58); -> 2Note that the return value is converted to a
BIGINT
!
ROUND(X,D)
X
, rounded to a number with D
decimals.
If D
is 0
, the result will have no decimal point or fractional
part.
mysql> select ROUND(1.298, 1); -> 1.3 mysql> select ROUND(1.298, 0); -> 1Note that the return value is converted to a
BIGINT
!
EXP(X)
e
(the base of natural logarithms) raised to
the power of X
.
mysql> select EXP(2); -> 7.389056 mysql> select EXP(-2); -> 0.135335
LOG(X)
X
.
mysql> select LOG(2); -> 0.693147 mysql> select LOG(-2); -> NULLIf you want the log of a number
X
to some arbitary base B
, use
the formula LOG(X)/LOG(B)
.
LOG10(X)
X
.
mysql> select LOG10(2); -> 0.301030 mysql> select LOG10(100); -> 2.000000 mysql> select LOG10(-100); -> NULL
POW(X,Y)
POWER(X,Y)
X
raised to the power of Y
.
mysql> select POW(2,2); -> 4.000000 mysql> select POW(2,-2); -> 0.250000
SQRT(X)
X
.
mysql> select SQRT(4); -> 2.000000 mysql> select SQRT(20); -> 4.472136
PI()
mysql> select PI(); -> 3.141593
COS(X)
X
, where X
is given in radians.
mysql> select COS(PI()); -> -1.000000
SIN(X)
X
, where X
is given in radians.
mysql> select SIN(PI()); -> 0.000000
TAN(X)
X
, where X
is given in radians.
mysql> select TAN(PI()+1); -> 1.557408
ACOS(X)
X
, that is, the value whose cosine is
X
. Returns NULL
if X
is not in the range -1
to
1
.
mysql> select ACOS(1); -> 0.000000 mysql> select ACOS(1.0001); -> NULL mysql> select ACOS(0); -> 1.570796
ASIN(X)
X
, that is, the value whose sine is
X
. Returns NULL
if X
is not in the range -1
to
1
.
mysql> select ASIN(0.2); -> 0.201358 mysql> select ASIN('foo'); -> 0.000000
ATAN(X)
X
, that is, the value whose tangent is
X
.
mysql> select ATAN(2); -> 1.107149 mysql> select ATAN(-2); -> -1.107149
ATAN2(X,Y)
X
and Y
. It is
similar to calculating the arc tangent of Y / X
, except that the
signs of both arguments are used to determine the quadrant of the
result.
mysql> select ATAN(-2,2); -> -0.785398 mysql> select ATAN(PI(),0); -> 1.570796
COT(X)
X
.
mysql> select COT(12); -> -1.57267341 mysql> select COT(0); -> NULL
RAND()
RAND(N)
0
to 1.0
.
If an integer argument N
is specified, it is used as the seed value.
mysql> select RAND(); -> 0.5925 mysql> select RAND(20); -> 0.1811 mysql> select RAND(20); -> 0.1811 mysql> select RAND(); -> 0.2079 mysql> select RAND(); -> 0.7888You can't use a column with
RAND()
values in an ORDER BY
clause, because ORDER BY
would evaluate the column multiple times.
In MySQL 3.23, you can however do:
SELECT * FROM table_name ORDER BY RAND()
This is useful to get a random sample of a set SELECT * FROM
table1,table2 WHERE a=b AND c<d ORDER BY RAND() LIMIT 1000
.
Note that a RAND()
in a WHERE
clause will be re-evaluated
every time the WHERE
is executed.
LEAST(X,Y,...)
INTEGER
context, or all arguments
are integer-valued, they are compared as integers.
REAL
context, or all arguments are
real-valued, they are compared as reals.
mysql> select LEAST(2,0); -> 0 mysql> select LEAST(34.0,3.0,5.0,767.0); -> 3.0 mysql> select LEAST("B","A","C"); -> "A"In MySQL versions prior to 3.22.5, you can use
MIN()
instead
of LEAST
.
GREATEST(X,Y,...)
LEAST
.
mysql> select GREATEST(2,0); -> 2 mysql> select GREATEST(34.0,3.0,5.0,767.0); -> 767.0 mysql> select GREATEST("B","A","C"); -> "C"In MySQL versions prior to 3.22.5, you can use
MAX()
instead
of GREATEST
.
DEGREES(X)
X
, converted from radians to degrees.
mysql> select DEGREES(PI()); -> 180.000000
RADIANS(X)
X
, converted from degrees to radians.
mysql> select RADIANS(90); -> 1.570796
TRUNCATE(X,D)
X
, truncated to D
decimals. If D
is 0
, the result will have no decimal point or fractional part.
mysql> select TRUNCATE(1.223,1); -> 1.2 mysql> select TRUNCATE(1.999,1); -> 1.9 mysql> select TRUNCATE(1.999,0); -> 1
String-valued functions return NULL
if the length of the result would
be greater than the max_allowed_packet
server parameter. See section 10.2.3 Tuning server parameters.
For functions that operate on string positions, the first position is numbered 1.
ASCII(str)
str
. Returns 0
if str
is the empty string. Returns
NULL
if str
is NULL
.
mysql> select ASCII('2'); -> 50 mysql> select ASCII(2); -> 50 mysql> select ASCII('dx'); -> 100
CONV(N,from_base,to_base)
N
, converted from base from_base
to base to_base
. Returns NULL
if any argument is NULL
.
The argument N
is interpreted as an integer, but may be specified as
an integer or a string. The minimum base is 2
and the maximum base is
36
. If to_base
is a negative number, N
is regarded as a
signed number. Otherwise, N
is treated as unsigned. CONV
works
with 64-bit precision.
mysql> select CONV("a",16,2); -> '1010' mysql> select CONV("6E",18,8); -> '172' mysql> select CONV(-17,10,-18); -> '-H' mysql> select CONV(10+"10"+'10'+0xa,10,10); -> '40'
BIN(N)
N
, where
N
is a longlong (BIGINT
) number. This is equivalent to
CONV(N,10,2)
. Returns NULL
if N
is NULL
.
mysql> select BIN(12); -> '1100'
OCT(N)
N
, where
N
is a longlong number. This is equivalent to CONV(N,10,8)
.
Returns NULL
if N
is NULL
.
mysql> select OCT(12); -> '14'
HEX(N)
N
, where
N
is a longlong (BIGINT
) number. This is equivalent to
CONV(N,10,16)
. Returns NULL
if N
is NULL
.
mysql> select HEX(255); -> 'FF'
CHAR(N,...)
CHAR()
interprets the arguments as integers and returns a string
consisting of the characters given by the ASCII code values of those
integers. NULL
values are skipped.
mysql> select CHAR(77,121,83,81,'76'); -> 'MySQL' mysql> select CHAR(77,77.3,'77.3'); -> 'MMM'
CONCAT(str1,str2,...)
NULL
if any argument is NULL
. May have more than 2 arguments.
A numeric argument is converted to the equivalent string form.
mysql> select CONCAT('My', 'S', 'QL'); -> 'MySQL' mysql> select CONCAT('My', NULL, 'QL'); -> NULL mysql> select CONCAT(14.3); -> '14.3'
LENGTH(str)
OCTET_LENGTH(str)
CHAR_LENGTH(str)
CHARACTER_LENGTH(str)
str
.
mysql> select LENGTH('text'); -> 4 mysql> select OCTET_LENGTH('text'); -> 4
LOCATE(substr,str)
POSITION(substr IN str)
substr
in string str
. Returns 0
if substr
is not in str
.
mysql> select LOCATE('bar', 'foobarbar'); -> 4 mysql> select LOCATE('xbar', 'foobar'); -> 0
LOCATE(substr,str,pos)
substr
in
string str
, starting at position pos
.
Returns 0
if substr
is not in str
.
mysql> select LOCATE('bar', 'foobarbar',5); -> 7
INSTR(str,substr)
substr
in
string str
. This is the same as the two-argument form of
LOCATE()
, except that the arguments are swapped.
mysql> select INSTR('foobarbar', 'bar'); -> 4 mysql> select INSTR('xbar', 'foobar'); -> 0
LPAD(str,len,padstr)
str
, left-padded with the string
padstr
until str
is len
characters long.
mysql> select LPAD('hi',4,'??'); -> '??hi'
RPAD(str,len,padstr)
str
, right-padded with the string
padstr
until str
is len
characters long.
mysql> select RPAD('hi',5,'?'); -> 'hi???'
LEFT(str,len)
len
characters from the string str
.
mysql> select LEFT('foobarbar', 5); -> 'fooba'
RIGHT(str,len)
len
characters from the string str
.
mysql> select RIGHT('foobarbar', 4); -> 'rbar'
SUBSTRING(str,pos,len)
SUBSTRING(str FROM pos FOR len)
MID(str,pos,len)
len
characters long from string str
,
starting at position pos
.
The variant form that uses FROM
is ANSI SQL92 syntax.
mysql> select SUBSTRING('Quadratically',5,6); -> 'ratica'
SUBSTRING(str,pos)
SUBSTRING(str FROM pos)
str
starting at position pos
.
mysql> select SUBSTRING('Quadratically',5); -> 'ratically' mysql> select SUBSTRING('foobarbar' FROM 4); -> 'barbar'
SUBSTRING_INDEX(str,delim,count)
str
after count
occurrences of the delimiter delim
.
If count
is positive, everything to the left of the final delimiter
(counting from the left) is returned.
If count
is negative, everything to the right of the final delimiter
(counting from the right) is returned.
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', 2); -> 'www.mysql' mysql> select SUBSTRING_INDEX('www.mysql.com', '.', -2); -> 'mysql.com'
LTRIM(str)
str
with leading space characters removed.
mysql> select LTRIM(' barbar'); -> 'barbar'
RTRIM(str)
str
with trailing space characters removed.
mysql> select RTRIM('barbar '); -> 'barbar'
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
str
with all remstr
prefixes and/or suffixes
removed. If none of the specifiers BOTH
, LEADING
or
TRAILING
are given, BOTH
is assumed. If remstr
is not
specified, spaces are removed.
mysql> select TRIM(' bar '); -> 'bar' mysql> select TRIM(LEADING 'x' FROM 'xxxbarxxx'); -> 'barxxx' mysql> select TRIM(BOTH 'x' FROM 'xxxbarxxx'); -> 'bar' mysql> select TRIM(TRAILING 'xyz' FROM 'barxxyz'); -> 'barx'
SOUNDEX(str)
str
. Two strings that sound ``about the
same'' should have identical soundex strings. A ``standard'' soundex string
is 4 characters long, but the SOUNDEX()
function returns an
arbitrarily long string. You can use SUBSTRING()
on the result to get
a ``standard'' soundex string. All non-alphanumeric characters are ignored
in the given string. All international alpha characters outside the A-Z range
are treated as vowels.
mysql> select SOUNDEX('Hello'); -> 'H400' mysql> select SOUNDEX('Quadratically'); -> 'Q36324'
SPACE(N)
N
space characters.
mysql> select SPACE(6); -> ' '
REPLACE(str,from_str,to_str)
str
with all all occurrences of the string
from_str
replaced by the string to_str
.
mysql> select REPLACE('www.mysql.com', 'w', 'Ww'); -> 'WwWwWw.mysql.com'
REPEAT(str,count)
str
repeated count
times. If count <= 0
, returns an empty string. Returns NULL
if
str
or count
are NULL
.
mysql> select REPEAT('MySQL', 3); -> 'MySQLMySQLMySQL'
REVERSE(str)
str
with the order of the characters reversed.
mysql> select REVERSE('abc'); -> 'cba'
INSERT(str,pos,len,newstr)
str
, with the substring beginning at position
pos
and len
characters long replaced by the string
newstr
.
mysql> select INSERT('Quadratic', 3, 4, 'What'); -> 'QuWhattic'
ELT(N,str1,str2,str3,...)
str1
if N
= 1
, str2
if N
=
2
, and so on. Returns NULL
if N
is less than 1
or greater than the number of arguments. ELT()
is the complement of
FIELD()
.
mysql> select ELT(1, 'ej', 'Heja', 'hej', 'foo'); -> 'ej' mysql> select ELT(4, 'ej', 'Heja', 'hej', 'foo'); -> 'foo'
FIELD(str,str1,str2,str3,...)
str
in the str1
, str2
,
str3
, ...
list.
Returns 0
if str
is not found.
FIELD()
is the complement of ELT()
.
mysql> select FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo'); -> 2 mysql> select FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo'); -> 0
FIND_IN_SET(str,strlist)
1
to N
if the string str
is in the list
strlist
consisting of N
substrings. A string list is a string
composed of substrings separated by `,' characters. If the first
argument is a constant string and the second is a column of type SET
,
the FIND_IN_SET()
function is optimized to use bit arithmetic!
Returns 0
if str
is not in strlist
or if strlist
is the empty string. Returns NULL
if either argument is NULL
.
This function will not work properly if the first argument contains a
`,'.
mysql> SELECT FIND_IN_SET('b','a,b,c,d'); -> 2
MAKE_SET(bits,str1,str2,...)
bits
set. str1
corresponds to bit 0, str2
to bit 1,
etc. NULL
strings in str1
, str2
, ...
are not appended to the result.
mysql> SELECT MAKE_SET(1,'a','b','c'); -> 'a' mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world'); -> 'hello,world' mysql> SELECT MAKE_SET(0,'a','b','c'); -> ''
EXPORT_SET(bits,on,off,[separator,[number_of_bits]])
mysql> select EXPORT_SET(5,'Y','N',',',4) -> Y,N,Y,N
LCASE(str)
LOWER(str)
str
with all characters changed to lowercase
according to the current character set mapping (the default is ISO-8859-1
Latin1).
mysql> select LCASE('QUADRATICALLY'); -> 'quadratically'
UCASE(str)
UPPER(str)
str
with all characters changed to uppercase
according to the current character set mapping (the default is ISO-8859-1
Latin1).
mysql> select UCASE('Hej'); -> 'HEJ'
LOAD_FILE(file_name)
max_allowed_packet
.
If the file doesn't exist or can't be read due to one of the above reasons,
the function returns NULL
.
mysql> UPDATE table_name SET blob_column=LOAD_FILE("/tmp/picture") WHERE id=1;
MySQL automatically converts numbers to strings as necessary, and vice versa:
mysql> SELECT 1+"1"; -> 2 mysql> SELECT CONCAT(2,' test'); -> '2 test'
If you want to convert a number to a string explicitly, pass it as the
argument to CONCAT()
.
If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This only affects comparisons.
See section 7.3.6 Date and time types for a description of the range of values each type has, and the valid formats in which date and time values may be specified.
Here is an example that uses date functions. The query below selects
all records with a date_col
value from within the last 30 days:
mysql> SELECT something FROM table WHERE TO_DAYS(NOW()) - TO_DAYS(date_col) <= 30;
DAYOFWEEK(date)
date
(1
= Sunday, 2
= Monday, ... 7
= Saturday).
These index values correspond to the ODBC standard.
mysql> select DAYOFWEEK('1998-02-03'); -> 3
WEEKDAY(date)
date
(0
= Monday, 1
= Tuesday, ... 6
= Sunday).
mysql> select WEEKDAY('1997-10-04 22:23:00'); -> 5 mysql> select WEEKDAY('1997-11-05'); -> 2
DAYOFMONTH(date)
date
, in the range 1
to
31
.
mysql> select DAYOFMONTH('1998-02-03'); -> 3
DAYOFYEAR(date)
date
, in the range 1
to
366
.
mysql> select DAYOFYEAR('1998-02-03'); -> 34
MONTH(date)
date
, in the range 1
to 12
.
mysql> select MONTH('1998-02-03'); -> 2
DAYNAME(date)
date
.
mysql> select DAYNAME("1998-02-05"); -> 'Thursday'
MONTHNAME(date)
date
.
mysql> select MONTHNAME("1998-02-05"); -> 'February'
QUARTER(date)
date
, in the range 1
to 4
.
mysql> select QUARTER('98-04-01'); -> 2
WEEK(date)
WEEK(date,first)
date
, in the range
0
to 52
, for locations where Sunday is the first day of the week.
The two-argument form of WEEK()
allows you to specify whether the
week starts on Sunday or Monday. The week starts on Sunday if the second
argument is 0
, on Monday if the second argument is 1
.
mysql> select WEEK('1998-02-20'); -> 7 mysql> select WEEK('1998-02-20',0); -> 7 mysql> select WEEK('1998-02-20',1); -> 8
YEAR(date)
date
, in the range 1000
to 9999
.
mysql> select YEAR('98-02-03'); -> 1998
HOUR(time)
time
, in the range 0
to 23
.
mysql> select HOUR('10:05:03'); -> 10
MINUTE(time)
time
, in the range 0
to 59
.
mysql> select MINUTE('98-02-03 10:05:03'); -> 5
SECOND(time)
time
, in the range 0
to 59
.
mysql> select SECOND('10:05:03'); -> 3
PERIOD_ADD(P,N)
N
months to period P
(in the format YYMM
or
YYYYMM
). Returns a value in the format YYYYMM
.
Note that the period argument P
is not a date value.
mysql> select PERIOD_ADD(9801,2); -> 199803
PERIOD_DIFF(P1,P2)
P1
and P2
.
P1
and P2
should be in the format YYMM
or YYYYMM
.
Note that the period arguments P1
and P2
are not
date values.
mysql> select PERIOD_DIFF(9802,199703); -> 11
DATE_ADD(date,INTERVAL expr type)
DATE_SUB(date,INTERVAL expr type)
ADDDATE(date,INTERVAL expr type)
SUBDATE(date,INTERVAL expr type)
ADDDATE()
and SUBDATE()
are synonyms for
DATE_ADD()
and DATE_SUB()
.
In MySQL 3.23, you can use +
and -
instead of
DATE_ADD()
and DATE_SUB()
. (See example)
date
is a DATETIME
or DATE
value specifying the starting
date. expr
is an expression specifying the interval value to be added
or substracted from the starting date. expr
is a string; it may start
with a `-' for negative intervals. type
is a keyword indicating
how the expression should be interpreted.
The EXTRACT(type FROM date)
function returns the 'type'
interval from the date.
The following table shows how the type
and expr
arguments
are related:
type value | Meaning | Expected expr format
|
SECOND | Seconds | SECONDS
|
MINUTE | Minutes | MINUTES
|
HOUR | Hours | HOURS
|
DAY | Days | DAYS
|
MONTH | Months | MONTHS
|
YEAR | Years | YEARS
|
MINUTE_SECOND | Minutes and seconds | "MINUTES:SECONDS"
|
HOUR_MINUTE | Hours and minutes | "HOURS:MINUTES"
|
DAY_HOUR | Days and hours | "DAYS HOURS"
|
YEAR_MONTH | Years and months | "YEARS-MONTHS"
|
HOUR_SECOND | Hours, minutes, | "HOURS:MINUTES:SECONDS"
|
DAY_MINUTE | Days, hours, minutes | "DAYS HOURS:MINUTES"
|
DAY_SECOND | Days, hours, minutes, seconds | "DAYS HOURS:MINUTES:SECONDS"
|
expr
format.
The ones shown in the table are the suggested delimiters. If the date
argument is a DATE
value and your calculations involve only
YEAR
, MONTH
and DAY
parts (that is, no time parts), the
result is a DATE
value. Otherwise the result is a DATETIME
value.
mysql> SELECT "1997-12-31 23:59:59" + INTERVAL 1 SECOND; -> 1998-01-01 00:00:00 mysql> SELECT INTERVAL 1 DAY + "1997-12-31"; -> 1998-01-01 mysql> SELECT "1998-01-01" - INTERVAL 1 SECOND; -> 1997-12-31 23:59:59 mysql> SELECT DATE_ADD("1997-12-31 23:59:59", INTERVAL 1 SECOND); -> 1998-01-01 00:00:00 mysql> SELECT DATE_ADD("1997-12-31 23:59:59", INTERVAL 1 DAY); -> 1998-01-01 23:59:59 mysql> SELECT DATE_ADD("1997-12-31 23:59:59", INTERVAL "1:1" MINUTE_SECOND); -> 1998-01-01 00:01:00 mysql> SELECT DATE_SUB("1998-01-01 00:00:00", INTERVAL "1 1:1:1" DAY_SECOND); -> 1997-12-30 22:58:59 mysql> SELECT DATE_ADD("1998-01-01 00:00:00", INTERVAL "-1 10" DAY_HOUR); -> 1997-12-30 14:00:00 mysql> SELECT DATE_SUB("1998-01-02", INTERVAL 31 DAY); -> 1997-12-02 mysql> SELECT EXTRACT(YEAR FROM "1999-07-02"); -> 1999 mysql> SELECT EXTRACT(YEAR_MONTH FROM "1999-07-02 01:02:03"); -> 199907 mysql> SELECT EXTRACT(DAY_MINUTE FROM "1999-07-02 01:02:03"); -> 20102If you specify an interval value that is too short (does not include all the interval parts that would be expected from the
type
keyword),
MySQL assumes you have left out the leftmost parts of the interval
value. For example, if you specify a type
of DAY_SECOND
, the
value of expr
is expected to have days, hours, minutes and seconds
parts. If you specify a value like "1:10"
, MySQL assumes
that the days and hours parts are missing and the value represents minutes
and seconds. In other words, "1:10" DAY_SECOND
is interpreted in such
a way that it is equivalent to "1:10" MINUTE_SECOND
. This is
analogous to the way that MySQL interprets TIME
values
as representing elapsed time rather than as time of day.
If you use incorrect dates, the result is NULL
. If you add
MONTH
, YEAR_MONTH
or YEAR
and the resulting date
has a day that is larger than the maximum day for the new month, the day is
adjusted to the maximum days in the new month.
mysql> select DATE_ADD('1998-01-30', Interval 1 month); -> 1998-02-28Note from the preceding example that the word
INTERVAL
and the
type
keyword are not case sensitive.
TO_DAYS(date)
date
, returns a daynumber (the number of days since year
0).
mysql> select TO_DAYS(950501); -> 728779 mysql> select TO_DAYS('1997-10-07'); -> 729669
TO_DAYS()
is not intended for use with values that precede the advent
of the Gregorian calendar (1582).
FROM_DAYS(N)
N
, returns a DATE
value.
mysql> select FROM_DAYS(729669); -> '1997-10-07'
FROM_DAYS()
is not intended for use with values that precede the
advent of the Gregorian calendar (1582).
DATE_FORMAT(date,format)
date
value according to the format
string. The
following specifiers may be used in the format
string:
%M | Month name (January ..December )
|
%W | Weekday name (Sunday ..Saturday )
|
%D | Day of the month with english suffix (1st , 2nd , 3rd , etc.)
|
%Y | Year, numeric, 4 digits |
%y | Year, numeric, 2 digits |
%a | Abbreviated weekday name (Sun ..Sat )
|
%d | Day of the month, numeric (00 ..31 )
|
%e | Day of the month, numeric (0 ..31 )
|
%m | Month, numeric (01 ..12 )
|
%c | Month, numeric (1 ..12 )
|
%b | Abbreviated month name (Jan ..Dec )
|
%j | Day of year (001 ..366 )
|
%H | Hour (00 ..23 )
|
%k | Hour (0 ..23 )
|
%h | Hour (01 ..12 )
|
%I | Hour (01 ..12 )
|
%l | Hour (1 ..12 )
|
%i | Minutes, numeric (00 ..59 )
|
%r | Time, 12-hour (hh:mm:ss [AP]M )
|
%T | Time, 24-hour (hh:mm:ss )
|
%S | Seconds (00 ..59 )
|
%s | Seconds (00 ..59 )
|
%p | AM or PM
|
%w | Day of the week (0 =Sunday..6 =Saturday)
|
%U | Week (0 ..52 ), where Sunday is the first day of the week
|
%u | Week (0 ..52 ), where Monday is the first day of the week
|
%% | A literal `%'. |
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y'); -> 'Saturday October 1997' mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s'); -> '22:23:00' mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%D %y %a %d %m %b %j'); -> '4th 97 Sat 04 10 Oct 277' mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H %k %I %r %T %S %w'); -> '22 22 10 10:23:00 PM 22:23:00 00 6'As of MySQL 3.23, the
%
is required before a format specifier
characters. In earlier versions of MySQL, %
was optional.
TIME_FORMAT(time,format)
DATE_FORMAT()
function above, but the
format
string may contain only those format specifiers that handle
hours, minutes and seconds. Other specifiers produce a NULL
value or
0
.
CURDATE()
CURRENT_DATE
'YYYY-MM-DD'
or YYYYMMDD
format, depending on whether the function is used in a string or numeric
context.
mysql> select CURDATE(); -> '1997-12-15' mysql> select CURDATE() + 0; -> 19971215
CURTIME()
CURRENT_TIME
'HH:MM:SS'
or HHMMSS
format, depending on whether the function is used in a string or numeric
context.
mysql> select CURTIME(); -> '23:50:26' mysql> select CURTIME() + 0; -> 235026
NOW()
SYSDATE()
CURRENT_TIMESTAMP
'YYYY-MM-DD HH:MM:SS'
or YYYYMMDDHHMMSS
format, depending on whether the function is used in
a string or numeric context.
mysql> select NOW(); -> '1997-12-15 23:50:26' mysql> select NOW() + 0; -> 19971215235026
UNIX_TIMESTAMP()
UNIX_TIMESTAMP(date)
'1970-01-01 00:00:00'
GMT). If UNIX_TIMESTAMP()
is called with
a date
argument, it returns the value of the argument as seconds since
'1970-01-01 00:00:00'
GMT. date
may be a DATE
string,
a DATETIME
string, a TIMESTAMP
, or a number in the format
YYMMDD
or YYYYMMDD
in local time.
mysql> select UNIX_TIMESTAMP(); -> 882226357 mysql> select UNIX_TIMESTAMP('1997-10-04 22:23:00'); -> 875996580When
UNIX_TIMESTAMP
is used on a TIMESTAMP
column, the function
will receive the value directly, with no implicit
``string-to-unix-timestamp'' conversion.
FROM_UNIXTIME(unix_timestamp)
unix_timestamp
argument as a value in
'YYYY-MM-DD HH:MM:SS'
or YYYYMMDDHHMMSS
format, depending on
whether the function is used in a string or numeric context.
mysql> select FROM_UNIXTIME(875996580); -> '1997-10-04 22:23:00' mysql> select FROM_UNIXTIME(875996580) + 0; -> 19971004222300
FROM_UNIXTIME(unix_timestamp,format)
format
string. format
may contain the same specifiers as
those listed in the entry for the DATE_FORMAT()
function.
mysql> select FROM_UNIXTIME(UNIX_TIMESTAMP(), '%Y %D %M %h:%i:%s %x'); -> '1997 23rd December 03:43:30 x'
SEC_TO_TIME(seconds)
seconds
argument, converted to hours, minutes and seconds,
as a value in 'HH:MM:SS'
or HHMMSS
format, depending on whether
the function is used in a string or numeric context.
mysql> select SEC_TO_TIME(2378); -> '00:39:38' mysql> select SEC_TO_TIME(2378) + 0; -> 3938
TIME_TO_SEC(time)
time
argument, converted to seconds.
mysql> select TIME_TO_SEC('22:23:00'); -> 80580 mysql> select TIME_TO_SEC('00:39:38'); -> 2378
DATABASE()
mysql> select DATABASE(); -> 'test'If there is no current database,
DATABASE()
returns the empty string.
USER()
SYSTEM_USER()
SESSION_USER()
mysql> select USER(); -> 'davida@localhost'In MySQL 3.22.11 or later, this includes the client hostname as well as the username. You can extract just the username part like this (which works whether or not the value includes a hostname part):
mysql> select substring_index(USER(),"@",1); -> 'davida'
PASSWORD(str)
str
. This is
the function that is used for encrypting MySQL passwords for storage
in the Password
column of the user
grant table.
mysql> select PASSWORD('badpwd'); -> '7f84554057dd964b'
PASSWORD()
encryption is non-reversible.
PASSWORD()
does not perform password encryption in the same way that
Unix passwords are encrypted. You should not assume that if your Unix
password and your MySQL password are the same, PASSWORD()
will result in the same encrypted value as is stored in the Unix password
file. See ENCRYPT()
.
ENCRYPT(str[,salt])
str
using the Unix crypt()
system call. The
salt
argument should be a string with two characters.
(As of MySQL 3.22.16, salt
may be longer than two characters.)
mysql> select ENCRYPT("hello"); -> 'VxuFAJXVARROc'If
crypt()
is not available on your system, ENCRYPT()
always
returns NULL
.
ENCRYPT()
ignores all but the first 8 characters of str
, at
least on some systems. This will be determined by the behavior of the
underlying crypt()
system call.
ENCODE(str,pass_str)
str
using pass_str
as the password.
To decrypt the result, use DECODE()
.
The results is a binary string. If you want to save it in a column,
use a BLOB
column type.
DECODE(crypt_str,pass_str)
crypt_str
using pass_str
as the
password. crypt_str
should be a string returned from
ENCODE()
.
MD5(string)
mysql> select MD5("testing") -> 'ae2b1fca515949e5d54fb22b8ed95575'This is a "RSA Data Security, Inc. MD5 Message-Digest Algorithm".
LAST_INSERT_ID([expr])
AUTO_INCREMENT
column.
See section 20.4.29 mysql_insert_id()
.
mysql> select LAST_INSERT_ID(); -> 195The last ID that was generated is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another
AUTO_INCREMENT
column with a
non-magic value (that is, a value that is not NULL
and not 0
).
If expr
is given as an argument to LAST_INSERT_ID()
in an
UPDATE
clause, then the value of the argument is returned as a
LAST_INSERT_ID()
value. This can be used to simulate sequences:
First create the table:
mysql> create table sequence (id int not null); mysql> insert into sequence values (0);Then the table can be used to generate sequence numbers like this:
mysql> update sequence set id=LAST_INSERT_ID(id+1);You can generate sequences without calling
LAST_INSERT_ID()
, but the
utility of using the function this way is that the ID value is maintained in
the server as the last automatically generated value. You can retrieve the
new ID as you would read any normal AUTO_INCREMENT
value in
MySQL. For example, LAST_INSERT_ID()
(without an argument)
will return the new ID. The C API function mysql_insert_id()
can also be used to get the value.
FORMAT(X,D)
X
to a format like '#,###,###.##'
, rounded
to D
decimals. If D
is 0
, the result will have no
decimal point or fractional part.
mysql> select FORMAT(12332.123456, 4); -> '12,332.1235' mysql> select FORMAT(12332.1,4); -> '12,332.1000' mysql> select FORMAT(12332.2,0); -> '12,332'
VERSION()
mysql> select VERSION(); -> '3.22.19b-log'
GET_LOCK(str,timeout)
str
, with a
timeout of timeout
seconds. Returns 1
if the lock was obtained
successfully, 0
if the attempt timed out, or NULL
if an error
occurred (such as running out of memory or the thread was killed with
mysqladmin kill
). A lock is released when you execute
RELEASE_LOCK()
, execute a new GET_LOCK()
or the thread
terminates. This function can be used to implement application locks or to
simulate record locks. It blocks requests by other clients for locks with
the same name; clients that agree on a given lock string name can use the
string to perform cooperative advisory locking.
mysql> select GET_LOCK("lock1",10); -> 1 mysql> select GET_LOCK("lock2",10); -> 1 mysql> select RELEASE_LOCK("lock2"); -> 1 mysql> select RELEASE_LOCK("lock1"); -> NULLNote that the second
RELEASE_LOCK()
call returns NULL
because
the lock "lock1"
was automatically released by the second
GET_LOCK()
call.
RELEASE_LOCK(str)
str
that was obtained with
GET_LOCK()
. Returns 1
if the lock was released, 0
if the
lock wasn't locked by this thread (in which case the lock is not released)
and NULL
if the named lock didn't exist. The lock will not exist if
it was never obtained by a call to GET_LOCK()
or if it already has
been released.
BENCHMARK(count,expr)
BENCHMARK()
function executes the expression expr
repeatedly count
times. It may be used to time how fast MySQL
processes the expression. The result value is always 0
. The intended
use is in the mysql
client, which reports query execution times.
mysql> select BENCHMARK(1000000,encode("hello","goodbye")); +----------------------------------------------+ | BENCHMARK(1000000,encode("hello","goodbye")) | +----------------------------------------------+ | 0 | +----------------------------------------------+ 1 row in set (4.74 sec)The time reported is elapsed time on the client end, not CPU time on the server end. It may be advisable to execute
BENCHMARK()
several
times, and interpret the result with regard to how heavily loaded the
server machine is.
GROUP BY
clauses
If you use a group function in a statement containing no GROUP BY
clause, it is equivalent to grouping on all rows.
COUNT(expr)
NULL
values in the rows
retrieved by a SELECT
statement.
mysql> select student.student_name,COUNT(*) from student,course where student.student_id=course.student_id GROUP BY student_name;
COUNT(*)
is somewhat different in that it returns a count of
the number of rows retrieved, whether or not they contain NULL
values.
COUNT(*)
is optimized to
return very quickly if the SELECT
retrieves from one table, no
other columns are retrieved and there is no WHERE
clause.
For example:
mysql> select COUNT(*) from student;
COUNT(DISTINCT expr,[expr...])
mysql> select COUNT(DISTINCT results) from student;In MySQL you can get the number of distinct expressions combinations by giving a list of expressions. In ANSI SQL you would have to do a concatenation of all expressions inside
CODE(DISTINCT ..)
.
AVG(expr)
expr
.
mysql> select student_name, AVG(test_score) from student GROUP BY student_name;
MIN(expr)
MAX(expr)
expr
. MIN()
and
MAX()
may take a string argument; in such cases they return the
minimum or maximum string value.
mysql> select student_name, MIN(test_score), MAX(test_score) from student GROUP BY student_name;
SUM(expr)
expr
. Note that if the return set has no rows,
it returns NULL!
STD(expr)
STDDEV(expr)
expr
. This is an extension to
ANSI SQL.
The STDDEV()
form of this function is provided for Oracle compatability.
BIT_OR(expr)
OR
of all bits in expr
. The calculation is
performed with 64-bit (BIGINT
precision.
BIT_AND(expr)
AND
of all bits in expr
. The calculation is
performed with 64-bit (BIGINT
precision.
MySQL has extended the use of GROUP BY
. You can use columns or
calculations in the SELECT
expressions which don't appear in
the GROUP BY
part. This stands for any possible value for this
group. You can use this to get better performance by avoiding sorting and
grouping on unnecessary items. For example, you don't need to group on
customer.name
in the following query:
mysql> select order.custid,customer.name,max(payments) from order,customer where order.custid = customer.custid GROUP BY order.custid;
In ANSI SQL, you would have to add customer.name
to the GROUP
BY
clause. In MySQL, the name is redundant.
Don't use this feature if the columns you omit from the GROUP BY
part
aren't unique in the group!
In some cases, you can use MIN()
and MAX()
to obtain a specific
column value even if it isn't unique. The following gives the value of
column
from the row containing the smallest value in the sort
column:
substr(MIN(concat(sort,space(6-length(sort)),column),7,length(column)))
Note that if you are using MySQL 3.22 (or earlier) or if you
are trying to follow ANSI SQL, you can't use expressions in GROUP
BY
or ORDER BY
clauses. You can work around this limitation by
using an alias for the expression:
mysql> SELECT id,FLOOR(value/100) AS val FROM tbl_name GROUP BY id,val ORDER BY val;
In MySQL
3.23 you can do:
mysql> SELECT id,FLOOR(value/100) FROM tbl_name ORDER BY RAND();
CREATE DATABASE
syntaxCREATE DATABASE db_name
CREATE DATABASE
creates a database with the given name. Rules for
allowable database names are given in section 7.1.5 Database, table, index, column and alias names. An error occurs if
the database already exists.
Databases in MySQL are implemented as directories containing files
that correspond to tables in the database. Since there are no tables in a
database when it is initially created, the CREATE DATABASE
statement
only creates a directory under the MySQL data directory.
You can also create databases with mysqladmin
.
See section 12.1 Overview of the different MySQL programs.
DROP DATABASE
syntaxDROP DATABASE [IF EXISTS] db_name
DROP DATABASE
drops all tables in the database and deletes the
database. Be VERY careful with this command!
DROP DATABASE
returns the number of files that were removed from the
database directory. Normally, this is three times the number of tables,
since each table corresponds to a `.MYD' file, a `.MYI' file and a
`.frm' file.
In MySQL 3.22 or later, you can use the keywords IF EXISTS
to
prevent an error from occurring if the database doesn't exist.
You can also drop databases with mysqladmin
.
See section 12.1 Overview of the different MySQL programs.
CREATE TABLE
syntaxCREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)] [table_options] [select_statement] create_definition: col_name type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT] [PRIMARY KEY] [reference_definition] or PRIMARY KEY (index_col_name,...) or KEY [index_name] (index_col_name,...) or INDEX [index_name] (index_col_name,...) or UNIQUE [INDEX] [index_name] (index_col_name,...) or [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...) [reference_definition] or CHECK (expr) type: TINYINT[(length)] [UNSIGNED] [ZEROFILL] or SMALLINT[(length)] [UNSIGNED] [ZEROFILL] or MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL] or INT[(length)] [UNSIGNED] [ZEROFILL] or INTEGER[(length)] [UNSIGNED] [ZEROFILL] or BIGINT[(length)] [UNSIGNED] [ZEROFILL] or REAL[(length,decimals)] [UNSIGNED] [ZEROFILL] or DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL] or FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL] or DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL] or NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL] or CHAR(length) [BINARY] or VARCHAR(length) [BINARY] or DATE or TIME or TIMESTAMP or DATETIME or TINYBLOB or BLOB or MEDIUMBLOB or LONGBLOB or TINYTEXT or TEXT or MEDIUMTEXT or LONGTEXT or ENUM(value1,value2,value3,...) or SET(value1,value2,value3,...) index_col_name: col_name [(length)] reference_definition: REFERENCES tbl_name [(index_col_name,...)] [MATCH FULL | MATCH PARTIAL] [ON DELETE reference_option] [ON UPDATE reference_option] reference_option: RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT table_options: TYPE = {ISAM | MYISAM | HEAP} or AUTO_INCREMENT = # or AVG_ROW_LENGTH = # or CHECKSUM = {0 | 1} or COMMENT = "string" or MAX_ROWS = # or MIN_ROWS = # or PACK_KEYS = {0 | 1} or PASSWORD = "string" or DELAY_KEY_WRITE = {0 | 1} or ROW_FORMAT= { default | dynamic | static | compressed } select_statement: [IGNORE | REPLACE] SELECT ... (Some legal select statement)
CREATE TABLE
creates a table with the given name in the current database. Rules for
allowable table names are given in section 7.1.5 Database, table, index, column and alias names. An error occurs if
there is no current database or if the table already exists.
In MySQL 3.22 or later, the table name can be specified as
db_name.tbl_name
. This works whether or not there is a current
database.
In MySQL 3.23, you can use the TEMPORARY
keyword when you
create a table. A temporary table will automatically be deleted if a
connection dies and the name is per connection. This means that two different
connections can both use the same temporary table name without conflicting
with each other or with an existing table of the same name. (The existing table
is hidden until the temporary table is deleted).
In MySQL 3.23 or later, you can use the keywords IF NOT EXISTS
so that an error does not occur if the table already exists. Note that there
is no verification that the table structures are identical.
Each table tbl_name
is represented by some files in the database
directory. In the case of ISAM-type tables you will get:
File | Purpose |
tbl_name.frm | Table definition (form) file |
tbl_name.MYD | Data file |
tbl_name.MYI | Index file |
For more information on the properties of the various column types, see section 7.3 Column types.
NULL
nor NOT NULL
is specified, the column
is treated as though NULL
had been specified.
AUTO_INCREMENT
.
When you insert a value of NULL
(recommended) or 0
into an
AUTO_INCREMENT
column, the column is set to value+1
, where
value
is the largest value for the column currently in the table.
AUTO_INCREMENT
sequences begin with 1
.
See section 20.4.29 mysql_insert_id()
.
If you delete the row containing the maximum value for an AUTO_INCREMENT
column, the value will be reused. If you delete all rows in the table, the
sequence starts over.
Note: There can be only one AUTO_INCREMENT
column per table,
and it must be indexed.
To make MySQL compatible with some ODBC applications, you can find
the last inserted row with the following query:
SELECT * FROM tbl_name WHERE auto_col IS NULL
NULL
values are handled differently for TIMESTAMP
columns than
for other column types. You cannot store a literal NULL
in a
TIMESTAMP
column; setting the column to NULL
sets it to the
current date and time. Because TIMESTAMP
columns behave this way, the
NULL
and NOT NULL
attributes do not apply in the normal way and
are ignored if you specify them.
On the other hand, to make it easier for MySQL clients to use
TIMESTAMP
columns, the server reports that such columns may be
assigned NULL
values (which is true), even though TIMESTAMP
never actually will contain a NULL
value. You can see this when you
use DESCRIBE tbl_name
to get a description of your table.
Note that setting a TIMESTAMP
column to 0
is not the same
as setting it to NULL
, because 0
is a valid TIMESTAMP
value.
DEFAULT
value is specified for a column, MySQL
automatically assigns one.
If the column may take NULL
as a value, the default value is
NULL
.
If the column is declared as NOT NULL
, the default value depends on
the column type:
AUTO_INCREMENT
attribute, the default is 0
. For an AUTO_INCREMENT
column, the
default value is the next value in the sequence.
TIMESTAMP
, the default is the
appropriate ``zero'' value for the type. For the first TIMESTAMP
column in a table, the default value is the current date and time.
See section 7.3.6 Date and time types.
ENUM
, the default is the empty string.
For ENUM
, the default is the first enumeration value.
KEY
is a synonym for INDEX
.
UNIQUE
key can have only distinct values. An
error occurs if you try to add a new row with a key that matches an existing
row.
PRIMARY KEY
is an unique KEY
with the extra constraint
that all key columns must be defined as NOT NULL
. In MySQL
the key is named PRIMARY
. A table can have only one PRIMARY KEY
.
If you don't have a PRIMARY KEY
and some applications ask for the
PRIMARY KEY
in your tables, MySQL will return the first
UNIQUE
key, which doesn't have any NULL
columns, as the
PRIMARY KEY
.
PRIMARY KEY
can be a multiple-column index. However, you cannot
create a multiple-column index using the PRIMARY KEY
key attibute in a
column specification. Doing so will mark only that single column as primary.
You must use the PRIMARY KEY(index_col_name, ...)
syntax.
index_col_name
, with an optional suffix (_2
,
_3
, ...
) to make it unique. You can see index names for a
table using SHOW INDEX FROM tbl_name
.
See section 7.21 SHOW
syntax (Get information about tables, columns,...).
MyISAM
table type supports indexes on columns that can have
NULL
values. In other cases you must declare such columns
NOT NULL
or an error results.
col_name(length)
syntax, you can specify an index which
uses only a part of a CHAR
or VARCHAR
column. This can
make the index file much smaller.
See section 7.3.9 Column indexes.
MyISAM
table type supports indexing on BLOB
and
TEXT
columns. When putting an index on a BLOB
or TEXT
column you MUST always specify the length of the index:
CREATE TABLE test (blob_col BLOB, index(blob_col(10)));
ORDER BY
or GROUP BY
with a TEXT
or
BLOB
column, only the first max_sort_length
bytes are used.
See section 7.3.7.2 The BLOB
and TEXT
types.
FOREIGN KEY
, CHECK
and REFERENCES
clauses don't
actually do anything. The syntax for them is provided only for compatibility,
to make it easier to port code from other SQL servers and to run applications
that create tables with references.
See section 5.4 Functionality missing from MySQL.
NULL
column takes one bit extra, rounded up to the nearest byte.
row length = 1 + (sum of column lengths) + (number of NULL columns + 7)/8 + (number of variable-length columns)
table_options
and SELECT
options is only
implemented in MySQL 3.23 and above.
The different table types are:
ISAM | The original table handler |
MyISAM | The new binary portable table handler |
HEAP | The data for this table is only stored in memory |
AUTO_INCREMENT | The next auto_increment value you want to set for your table (MyISAM) |
AVG_ROW_LENGTH | An approximation of the average row length for your table. You only need to set this for tables with variable size records. |
CHECKSUM | Set this to 1 if you want MySQL to maintain a checksum for all rows (makes the table a little slower to update but makes it easier to find corrupted tables) (MyISAM) |
COMMENT | A 60 character comment for your table |
MAX_ROWS | Max number of rows you plan to store in the table |
MIN_ROWS | Minimum number of rows you plan to store in the table |
PACK_KEYS | Set this to 1 if you want to have smaller index. This usually makes updates slower and reads faster (MyISAM, ISAM). |
PASSWORD | Encrypt the .frm file with a password. This option doesn't do anything in the standard MySQL version.
|
DELAY_KEY_WRITE | Set this to 1 if want to delay key table updates until the table is closed (MyISAM). |
ROW_FORMAT | Defines how the rows should be stored (for the future). |
MyISAM
table, MySQL uses the product of
max_rows * avg_row_length
to decide how big the resulting table
will be. If you don't specify any of the above options, the maximum size
for a table will be 4G (or 2G if your operating systems only supports 2G
tables).
SELECT
after the CREATE STATEMENT
,
MySQL will create new fields for all elements in the
SELECT
. For example:
mysql> CREATE TABLE test (a int not null auto_increment, primary key (a), key(b)) TYPE=HEAP SELECT b,c from test2;This will create a
HEAP
table with 3 columns. Note that the table will
automatically be deleted if any errors occur while copying data
into the table.
In some cases, MySQL silently changes a column specification from
that given in a CREATE TABLE
statement. (This may also occur with
ALTER TABLE
.)
VARCHAR
columns with a length less than four are changed to
CHAR
.
VARCHAR
, TEXT
or BLOB
),
all CHAR
columns longer than three characters are changed to
VARCHAR
columnss. This doesn't affect how you use the columns in
any way; in MySQL, VARCHAR
is just a different way to
store characters. MySQL performs this conversion because it
saves space and makes table operations faster. See section 10.6 Choosing a table type.
TIMESTAMP
display sizes must be even and in the range from 2 to 14.
If you specify a display size of 0 or greater than 14, the size is coerced
to 14. Odd-valued sizes in the range from 1 to 13 are coerced
to the next higher even number.
NULL
in a TIMESTAMP
column; setting
it to NULL
sets it to the current date and time. Because
TIMESTAMP
columns behave this way, the NULL
and NOT NULL
attributes do not apply in the normal way and are ignored if you specify
them. DESCRIBE tbl_name
always reports that a TIMESTAMP
column may be assigned NULL
values.
If you want to see whether or not MySQL used a column type other
than the one you specified, issue a DESCRIBE tbl_name
statement after
creating or altering your table.
Certain other column type changes may occur if you compress a table
using myisampack
. See section 10.6.3 Compressed table characteristics.
ALTER TABLE
syntaxALTER [IGNORE] TABLE tbl_name alter_spec [, alter_spec ...] alter_specification: ADD [COLUMN] create_definition [FIRST | AFTER column_name ] or ADD INDEX [index_name] (index_col_name,...) or ADD PRIMARY KEY (index_col_name,...) or ADD UNIQUE [index_name] (index_col_name,...) or ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT} or CHANGE [COLUMN] old_col_name create_definition or MODIFY [COLUMN] create_definition or DROP [COLUMN] col_name or DROP PRIMARY KEY or DROP INDEX index_name or RENAME [AS] new_tbl_name or table_options
ALTER TABLE
allows you to change the structure of an existing table.
For example, you can add or delete columns, create or destroy indexes, change
the type of existing columns, or rename columns or the table itself. You can
also change the comment for the table and type of the table.
See section 7.7 CREATE TABLE
syntax.
If you use ALTER TABLE
to change a column specification but
DESCRIBE tbl_name
indicates that your column was not changed, it is
possible that MySQL ignored your modification for one of the reasons
described in section 7.7.1 Silent column specification changes. For example, if you try to change
a VARCHAR
column to CHAR
, MySQL will still use
VARCHAR
if the table contains other variable-length columns.
ALTER TABLE
works by making a temporary copy of the original table.
The alteration is performed on the copy, then the original table is
deleted and the new one is renamed. This is done in such a way that
all updates are automatically redirected to the new table without
any failed updates. While ALTER TABLE
is executing, the original
table is readable by other clients. Updates and writes to the table
are stalled until the new table is ready.
ALTER TABLE
, you need select, insert,
delete, update, create and drop
privileges on the table.
IGNORE
is a MySQL extension to ANSI SQL92.
It controls how ALTER TABLE
works if there are duplicates on
unique keys in the new table.
If IGNORE
isn't specified, the copy is aborted and rolled back.
If IGNORE
is specified, then for rows with duplicates on a unique
key, only the first row is used; the others are deleted.
ADD
, ALTER
, DROP
and
CHANGE
clauses in a single ALTER TABLE
statement. This is a
MySQL extension to ANSI SQL92, which allows only one of each clause
per ALTER TABLE
statement.
CHANGE col_name
, DROP col_name
and DROP
INDEX
are MySQL extensions to ANSI SQL92.
MODIFY
is an Oracle extension to ALTER TABLE
.
COLUMN
is a pure noise word and can be omitted.
ALTER TABLE tbl_name RENAME AS new_name
without any other
options, MySQL simply renames the files that correspond to the table
tbl_name
. There is no need to create the temporary table.
create_definition
clauses use the same syntax for ADD
and
CHANGE
as for CREATE TABLE
. Note that this syntax includes
the column name, not just the column type.
See section 7.7 CREATE TABLE
syntax.
CHANGE old_col_name create_definition
clause. To do so, specify the old and new column names and the type that
the column currently has. For example, to rename an INTEGER
column
from a
to b
, you can do this:
mysql> ALTER TABLE t1 CHANGE a b INTEGER;If you want to change a column's type but not the name,
CHANGE
syntax still requires two column names even if they are the same. For
example:
mysql> ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;However, as of MySQL 3.22.16a, you can also use
MODIFY
to
change a column's type without renaming it:
mysql> ALTER TABLE t1 MODIFY b BIGINT NOT NULL;
CHANGE
or MODIFY
to shorten a column for which
an index exists on part of the column (for instance, if you have an index
on the first 10 characters of a VARCHAR
column), you cannot make
the column shorter than the number of characters that are indexed.
CHANGE
or MODIFY
,
MySQL tries to convert data to the new type as well as possible.
FIRST
or ADD ...
AFTER col_name
to add a column at a specific position within a table row.
The default is to add the column last.
ALTER COLUMN
specifies a new default value for a column
or removes the old default value.
If the old default is removed and the column can be NULL
, the new
default is NULL
. If the column cannot be NULL
, MySQL
assigns a default value.
Default value assignment is described in
section 7.7 CREATE TABLE
syntax.
DROP INDEX
removes an index. This is a MySQL extension to
ANSI SQL92.
DROP PRIMARY KEY
drops the primary index. If no such
index exists, it drops the first UNIQUE
index in the table.
(MySQL marks the first UNIQUE
key as the PRIMARY KEY
if no PRIMARY KEY
was specified explicitly.)
mysql_info()
, you can find out how many
records were copied, and (when IGNORE
is used) how many records were
deleted due to duplication of unique key values.
FOREIGN KEY
, CHECK
and REFERENCES
clauses don't
actually do anything. The syntax for them is provided only for compatibility,
to make it easier to port code from other SQL servers and to run applications
that create tables with references.
See section 5.4 Functionality missing from MySQL.
Here is an example that shows some of the uses of ALTER TABLE
. We
begin with a table t1
that is created as shown below:
mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));
To rename the table from t1
to t2
:
mysql> ALTER TABLE t1 RENAME t2;
To change column a
from INTEGER
to TINYINT NOT NULL
(leaving the name the same), and to change column b
from
CHAR(10)
to CHAR(20)
as well as renaming it from b
to
c
:
mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);
To add a new TIMESTAMP
column named d
:
mysql> ALTER TABLE t2 ADD d TIMESTAMP;
To add an index on column d
, and make column a
the primary key:
mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);
To remove column c
:
mysql> ALTER TABLE t2 DROP COLUMN c;
To add a new AUTO_INCREMENT
integer column named c
:
mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT, ADD INDEX (c);
Note that we indexed c
, because AUTO_INCREMENT
columns must be
indexed, and also that we declare c
as NOT NULL
, because
indexed columns cannot be NULL
.
When you add an AUTO_INCREMENT
column, column values are filled in
with sequence numbers for you automatically.
OPTIMIZE TABLE
syntaxOPTIMIZE TABLE tbl_name
OPTIMZE TABLE
should be used if you have deleted a large part of a
table or if you have made many changes to a table with variable-length rows
(tables that have VARCHAR
, BLOB
or TEXT
columns).
Deleted records are maintained in a linked list and subsequent INSERT
operations reuse old record positions. You can use OPTIMIZE TABLE
to
reclaim the unused space.
OPTIMIZE TABLE
works by making a temporary copy of the original
table. The old table is copied to the new table (without the unused rows),
then the original table is deleted and the new one is renamed. This is done
in such a way that all updates are automatically redirected to the new table
without any failed updates. While OPTIMIZE TABLE
is executing, the
original table is readable by other clients. Updates and writes to the table
are stalled until the new table is ready.
DROP TABLE
syntaxDROP TABLE [IF EXISTS] tbl_name [, tbl_name,...]
DROP TABLE
removes one or more tables. All table data and the table
definition are removed, so be careful with this command!
In MySQL 3.22 or later, you can use the keywords IF EXISTS
to
prevent an error from occurring for tables that don't exist.
DELETE
syntaxDELETE [LOW_PRIORITY] FROM tbl_name [WHERE where_definition] [LIMIT rows]
DELETE
deletes rows from tbl_name
that satisfy the condition
given by where_definition
, and returns the number of records deleted.
If you issue a DELETE
with no WHERE
clause, all rows are
deleted. MySQL does this by recreating the table as an empty table,
which is much faster than deleting each row. In this case, DELETE
returns zero as the number of affected records. (MySQL can't return
the number of rows that were actually deleted, since the recreate is done
without opening the data files. As long as the table definition file
`tbl_name.frm' is valid, the table can be recreated this way, even if
the data or index files have become corrupted.).
If you really want to know how many records are deleted when you are deleting
all rows, and are willing to suffer a speed penalty, you can use a
DELETE
statement of this form:
mysql> DELETE FROM tbl_name WHERE 1>0;
Note that this is MUCH slower than DELETE FROM tbl_name
with no
WHERE
clause, because it deletes rows one at a time.
If you specify the keyword LOW_PRIORITY
, execution of the
DELETE
is delayed until no other clients are reading from the table.
Deleted records are maintained in a linked list and subsequent INSERT
operations reuse old record positions. To reclaim unused space and reduce
file sizes, use the OPTIMIZE TABLE
statement or the myisamchk
utility to reorganize tables. OPTIMIZE TABLE
is easier, but
myisamchk
is faster.
See section 7.9 OPTIMIZE TABLE
syntax, and
section 13.4.3 Table optimization.
The MySQL-specific LIMIT rows
option to DELETE
tells
the server the maximum number of rows to be deleted before control is
returned to the client. This can be used to ensure that a specific
DELETE
command doesn't take too much time. You can simply repeat
the DELETE
command until the number of affected rows is less than
the LIMIT
value.
SELECT
syntaxSELECT [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [HIGH_PRIORITY] [DISTINCT | DISTINCTROW | ALL] select_expression,... [INTO {OUTFILE | DUMPFILE} 'file_name' export_options] [FROM table_references [WHERE where_definition] [GROUP BY col_name,...] [HAVING where_definition] [ORDER BY {unsigned_integer | col_name | formula} [ASC | DESC] ,...] [LIMIT [offset,] rows] [PROCEDURE procedure_name] ]
SELECT
is used to retrieve rows selected from one or more tables.
select_expression
indicates the columns you want to retrieve.
SELECT
may also be used to retrieve rows computed without reference to
any table. For example:
mysql> SELECT 1 + 1; -> 2
All keywords used must be given in exactly the order shown above. For example,
a HAVING
clause must come after any GROUP BY
clause and before
any ORDER BY
clause.
SELECT
expression may be given an alias using AS
. The alias
is used as the expression's column name and can be used with
ORDER BY
or HAVING
clauses. For example:
mysql> select concat(last_name,', ',first_name) AS full_name from mytable ORDER BY full_name;
FROM table_references
clause indicates the tables from which to
retrieve rows. If you name more than one table, you are performing a
join. For information on join syntax, see section 7.13 JOIN
syntax.
col_name
, tbl_name.col_name
or
db_name.tbl_name.col_name
. You need not specify a tbl_name
or
db_name.tbl_name
prefix for a column reference in a SELECT
statement unless the reference would be ambiguous. See section 7.1.5 Database, table, index, column and alias names,
for examples of ambiguity that require the more explicit column reference
forms.
tbl_name [AS] alias_name
.
mysql> select t1.name, t2.salary from employee AS t1, info AS t2 where t1.name = t2.name; mysql> select t1.name, t2.salary from employee t1, info t2 where t1.name = t2.name;
ORDER BY
and
GROUP BY
clauses using column names, column aliases or column
positions. Column positions begin with 1.
mysql> select college, region, seed from tournament ORDER BY region, seed; mysql> select college, region AS r, seed AS s from tournament ORDER BY r, s; mysql> select college, region, seed from tournament ORDER BY 2, 3;To sort in reverse order, add the
DESC
(descending) keyword to the
name of the column in the ORDER BY
clause that you are sorting by.
The default is ascending order; this may be specified explicitly using
the ASC
keyword.
HAVING
clause can refer to any column or alias named in the
select_expression
. It is applied last, just before items are sent to
the client, with no optimization. Don't use HAVING
for items that
should be in the WHERE
clause. For example, do not write this:
mysql> select col_name from tbl_name HAVING col_name > 0;Write this instead:
mysql> select col_name from tbl_name WHERE col_name > 0;In MySQL 3.22.5 or later, you can also write queries like this:
mysql> select user,max(salary) from users group by user HAVING max(salary)>10;In older MySQL versions, you can write this instead:
mysql> select user,max(salary) AS sum from users group by user HAVING sum>10;
SQL_SMALL_RESULT
, SQL_BIG_RESULT
, STRAIGHT_JOIN
and
HIGH_PRIORITY
are MySQL extensions to ANSI SQL92.
STRAIGHT_JOIN
forces the optimizer to join the tables in the order in
which they are listed in the FROM
clause. You can use this to speed up
a query if the optimizer joins the tables in non-optimal order.
See section 7.22 EXPLAIN
syntax (Get information about a SELECT
).
SQL_SMALL_RESULT
can be used with GROUP BY
or DISTINCT
to tell the optimizer that the result set will be small. In this case,
MySQL will use fast temporary tables to store the resulting table
instead of using sorting. SQL_SMALL_RESULT
is a MySQL
SQL_BIG_RESULT
can be used with GROUP BY
or DISTINCT
to tell the optimizer that the result set will have many rows. In this case,
MySQL will directly use disk based temporary tables if needed.
MySQL
in this case will prefer to do a sort instead doing a
temporary table with a key on the GROUP BY
elements.
HIGH_PRIORITY
will give the SELECT
higher priority than
a statement that updates a table. You should only use this for queries
that are very fast and must be done at once. A SELECT HIGH_PRIORITY
query will run if the table is locked for read even if there is an update
statement that is waiting for the table to be free.
LIMIT
clause can be used to constrain the number of rows returned
by the SELECT
statement. LIMIT
takes one or two numeric
arguments.
If two arguments are given, the first specifies the offset of the first row to
return, the second specifies the maximum number of rows to return.
The offset of the initial row is 0 (not 1).
mysql> select * from table LIMIT 5,10; # Retrieve rows 6-15If one argument is given, it indicates the maximum number of rows to return.
mysql> select * from table LIMIT 5; # Retrieve first 5 rowsIn other words,
LIMIT n
is equivalent to LIMIT 0,n
.
SELECT ... INTO OUTFILE 'file_name'
form of SELECT
writes
the selected rows to a file. The file is created on the server host, and
cannot already exist (among other things, this prevents database tables and
files such as `/etc/passwd' from being destroyed). You must have the
file privilege on the server host to use this form of SELECT
.
SELECT ... INTO OUTFILE
is the complement of LOAD DATA
INFILE
; the syntax for the export_options
part of the statement
consists of the same FIELDS
and LINES
clauses that are used
with the LOAD DATA INFILE
statement.
See section 7.16 LOAD DATA INFILE
syntax.
In the resulting text file, only the following characters are escaped by
the ESCAPED BY
character:
ESCAPED BY
character
FIELDS TERMINATED BY
LINES TERMINATED BY
ASCII 0
is converted to ESCAPED BY
followed by 0
(ASCII 48
).
The reason for the above is that you MUST escape any FIELDS
TERMINATED BY
, ESCAPED BY
or LINES TERMINATED BY
characters to reliably be able to read the file
back. ASCII 0
is escaped to make it easier to view with some
pagers.
As the resulting file doesn't have to conform to the SQL syntax, nothing
else need be escaped.
If you use INTO DUMPFILE
instead of INTO OUTFILE
MySQL
will only write one row into the file, without any column or line
terminations and without any escaping. This is useful if you want to
store a blob in a file.
JOIN
syntax
MySQL supports the following JOIN
syntaxes for use in
SELECT
statements:
table_reference, table_reference table_reference [CROSS] JOIN table_reference table_reference INNER JOIN table_reference table_reference STRAIGHT_JOIN table_reference table_reference LEFT [OUTER] JOIN table_reference ON conditional_expr table_reference LEFT [OUTER] JOIN table_reference USING (column_list) table_reference NATURAL LEFT [OUTER] JOIN table_reference { oj table_reference LEFT OUTER JOIN table_reference ON conditional_expr }
The last LEFT OUTER JOIN
syntax shown above exists only for
compatibility with ODBC.
tbl_name AS alias_name
or
tbl_name alias_name
.
mysql> select t1.name, t2.salary from employee AS t1, info AS t2 where t1.name = t2.name;
INNER JOIN
and ,
(comma) are semantically equivalent.
Both do a full join between the tables used. Normally, you specify how
the tables should be linked in the WHERE
condition.
ON
conditional is any conditional of the form that may be used in
a WHERE
clause.
LEFT JOIN
, a
row with all columns set to NULL
is used for the right table. You can
use this fact to find records in a table that have no counterpart in another
table:
mysql> select table1.* from table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id is NULL;This example finds all rows in
table1
with an id
value that is
not present in table2
(i.e., all rows in table1
with no
corresponding row in table2
). This assumes that table2.id
is
declared NOT NULL
, of course.
USING
(column_list)
clause names a list of columns that must
exist in both tables. A USING
clause such as:
A LEFT JOIN B USING (C1,C2,C3,...)is defined to be semantically identical to an
ON
expression like
this:
A.C1=B.C1 AND A.C2=B.C2 AND A.C3=B.C3,...
NATURAL LEFT JOIN
of two tables is defined to be semantically
equivalent to a LEFT JOIN
with a USING
clause that names all
columns that exist in both tables.
STRAIGHT_JOIN
is identical to JOIN
, except that the left table
is always read before the right table. This can be used for those (few)
cases where the join optimizer puts the tables in the wrong order.
Some examples:
mysql> select * from table1,table2 where table1.id=table2.id; mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id; mysql> select * from table1 LEFT JOIN table2 USING (id); mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id LEFT JOIN table3 ON table2.id=table3.id;
See section 10.5.4 How MySQL optimizes LEFT JOIN
.
INSERT
syntaxINSERT [LOW_PRIORITY | DELAYED] [IGNORE] [INTO] tbl_name [(col_name,...)] VALUES (expression,...),(...),... or INSERT [LOW_PRIORITY | DELAYED] [IGNORE] [INTO] tbl_name [(col_name,...)] SELECT ... or INSERT [LOW_PRIORITY | DELAYED] [IGNORE] [INTO] tbl_name SET col_name=expression, col_name=expression, ...
INSERT
inserts new rows into an existing table. The INSERT ...
VALUES
form of the statement inserts rows based on explicitly-specified
values. The INSERT ... SELECT
form inserts rows selected from another
table or tables. The INSERT ... VALUES
form with multiple value lists
is supported in MySQL 3.22.5 or later. The
col_name=expression
syntax is supported in MySQL 3.22.10 or
later.
tbl_name
is the table into which rows should be inserted. The column
name list or the SET
clause indicates which columns the statement
specifies values for.
INSERT ... VALUES
or INSERT
... SELECT
, values for all columns must be provided in the
VALUES()
list or by the SELECT
. If you don't know the order of
the columns in the table, use DESCRIBE tbl_name
to find out.
CREATE TABLE
syntax.
expression
may refer to any column that was set earlier in a value
list. For example, you can say this:
mysql> INSERT INTO tbl_name (col1,col2) VALUES(15,col1*2);But not this:
mysql> INSERT INTO tbl_name (col1,col2) VALUES(col2*2,15);
LOW_PRIORITY
, execution of the
INSERT
is delayed until no other clients are reading from the table.
In this case the client has to wait until the insert statement is completed,
which may take a long time if the table is in heavy use. This is in
contrast to INSERT DELAYED
which lets the client continue at once.
IGNORE
in an INSERT
with many value
rows, any rows which duplicate an existing PRIMARY
or UNIQUE
key in the table are ignored and are not inserted. If you do not specify
IGNORE
, the insert is aborted if there is any row that duplicates an
existing key value. You can check with the C API function
mysql_info()
how many rows were inserted into the table.
DONT_USE_DEFAULT_FIELDS
option, INSERT
statements generate an error unless you explicitly
specify values for all columns that require a non-NULL
value.
See section 4.7.3 Typical configure
options.
INSERT INTO ... SELECT
statement:
ORDER BY
clause.
INSERT
statement cannot appear in the
FROM
clause of the SELECT
part of the query, because it's
forbidden in ANSI SQL to SELECT
from the same table into which you are
INSERT
ing. (The problem is that the SELECT
possibly would
find records that were inserted earlier during the same run. When using
sub-select clauses, the situation could easily be very confusing!)
AUTO_INCREMENT
columns work as usual.
If you use INSERT ... SELECT
or a INSERT ... VALUES
statement with multiple value lists, you can use the C API function
mysql_info()
to get information about the query. The format of the
information string is shown below:
Records: 100 Duplicates: 0 Warnings: 0
Duplicates
indicates the number of rows that couldn't be inserted
because they would duplicate some existing unique index value.
Warnings
indicates the number of attempts to insert column values that
were problematic in some way. Warnings can occur under any of the following
conditions:
NULL
into a column that has been declared NOT NULL
.
The column is set to its default value.
'10.34 a'
. The trailing
garbage is stripped and the remaining numeric part is inserted. If the value
doesn't make sense as a number at all, the column is set to 0
.
CHAR
, VARCHAR
, TEXT
or
BLOB
column that exceeds the column's maximum length. The value is
truncated to the column's maximum length.
The DELAYED
option
for the
INSERT
statement is a MySQL-specific option that is very
useful if you have clients that can't wait for the INSERT
to complete.
This is a common problem when you use MySQL for logging and you also
periodically run SELECT
statements that take a long time to complete.
DELAYED
was introduced in MySQL 3.22.15. It is a
MySQL extension to ANSI SQL92.
When you use INSERT DELAYED
, the client will get an ok at once
and the row will be inserted when the table is not in use by any other thread.
Another major benefit of using INSERT DELAYED
is that inserts
from many clients are bundled together and written in one block. This is much
faster than doing many separate inserts.
Note that currently the queued rows are only stored in memory until they are
inserted into the table. This means that if you kill mysqld
the hard way (kill -9
) or if mysqld
dies unexpectedly, any
queued rows that weren't written to disk are lost!
The following describes in detail what happens when you use the
DELAYED
option to INSERT
or REPLACE
. In this
description, the ``thread'' is the thread that received an INSERT
DELAYED
command and ``handler'' is the thread that handles all
INSERT DELAYED
statements for a particular table.
DELAYED
statement for a table, a handler
thread is created to process all DELAYED
statements for the table, if
no such handler already exists.
DELAYED
lock already; if not, it tells the handler thread to do so. The
DELAYED
lock can be obtained even if other threads have a READ
or WRITE
lock on the table. However, the handler will wait for all
ALTER TABLE
locks or FLUSH TABLES
to ensure that the table
structure is up to date.
INSERT
statement but instead of writing
the row to the table it puts a copy of the final row into a queue that
is managed by the handler thread. Any syntax errors are noticed by the
thread and reported to the client program.
AUTO_INCREMENT
value for the resulting row; it can't obtain them from the server, because
the INSERT
returns before the insert operation has been completed. If
you use the C API, the mysql_info()
function doesn't return anything
meaningful, for the same reason.
delayed_insert_limit
rows are written, the handler checks
whether or not any SELECT
statements are still pending. If so, it
allows these to execute before continuing.
INSERT DELAYED
commands are received within
delayed_insert_timeout
seconds, the handler terminates.
delayed_queue_size
rows are pending already in a specific
handler queue, the thread waits until there is room in the queue. This is
useful to ensure that the mysqld
server doesn't use all memory for the
delayed memory queue.
delayed_insert
in the Command
column. It will
be killed if you execute a FLUSH TABLES
command or kill it with
KILL thread_id
. However, it will first store all queued rows into the
table before exiting. During this time it will not accept any new
INSERT
commands from another thread. If you execute an INSERT
DELAYED
command after this, a new handler thread will be created.
INSERT DELAYED
commands have higher
priority than normal INSERT
commands if there is an INSERT
DELAYED
handler already running! Other update commands will have to wait
until the INSERT DELAY
queue is empty, someone kills the handler
thread (with KILL thread_id
) or someone executes FLUSH TABLES
.
INSERT
DELAYED
commands:
Delayed_insert_threads | Number of handler threads |
Delayed_writes | Number of rows written with INSERT DELAYED
|
Not_flushed_delayed_rows | Number of rows waiting to be written |
SHOW STATUS
statement or
by executing a mysqladmin extended-status
command.
Note that INSERT DELAYED
is slower than a normal INSERT if the
table is not in use. There is also the additional overhead for the
server to handle a separate thread for each table on which you use
INSERT DELAYED
. This means that you should only use INSERT
DELAYED
when you are really sure you need it!
REPLACE
syntaxREPLACE [LOW_PRIORITY | DELAYED] [INTO] tbl_name [(col_name,...)] VALUES (expression,...) or REPLACE [LOW_PRIORITY | DELAYED] [INTO] tbl_name [(col_name,...)] SELECT ... or REPLACE [LOW_PRIORITY | DELAYED] [INTO] tbl_name SET col_name=expression, col_name=expression,...
REPLACE
works exactly like INSERT
, except that if an old
record in the table has the same value as a new record on a unique index,
the old record is deleted before the new record is inserted.
See section 7.14 INSERT
syntax.
LOAD DATA INFILE
syntaxLOAD DATA [LOW_PRIORITY] [LOCAL] INFILE 'file_name.txt' [REPLACE | IGNORE] INTO TABLE tbl_name [FIELDS [TERMINATED BY '\t'] [OPTIONALLY] ENCLOSED BY ''] [ESCAPED BY '\\' ]] [LINES TERMINATED BY '\n'] [IGNORE number LINES] [(col_name,...)]
The LOAD DATA INFILE
statement reads rows from a text file into a
table at a very high speed. If the LOCAL
keyword is specified, the
file is read from the client host. If LOCAL
is not specified, the
file must be located on the server. (LOCAL
is available in
MySQL 3.22.6 or later.)
For security reasons, when reading text files located on the server, the
files must either reside in the database directory or be readable by all.
Also, to use LOAD DATA INFILE
on server files, you must have the
file privilege on the server host.
See section 6.5 Privileges provided by MySQL.
If you specify the keyword LOW_PRIORITY
, execution of the
LOAD DATA
statement is delayed until no other clients are reading
from the table.
Using LOCAL
will be a bit slower than letting the server access the
files directly, since the contents of the file must travel from the client
host to the server host. On the other hand, you do not need the
file privilege to load local files.
You can also load data files by using the mysqlimport
utility; it
operates by sending a LOAD DATA INFILE
command to the server. The
--local
option causes mysqlimport
to read data files from the
client host. You can specify the --compress
option to get better
performance over slow networks if the client and server support the
compressed protocol.
When locating files on the server host, the server uses the following rules:
Note that these rules mean a file given as `./myfile.txt' is read from
the server's data directory, whereas a file given as `myfile.txt' is
read from the database directory of the current database. Note also that
for statements such as those below, the file is read from the database
directory for db1
, not db2
:
mysql> USE db1; mysql> LOAD DATA INFILE "./data.txt" INTO TABLE db2.my_table;
The REPLACE
and IGNORE
keywords control handling of input
records that duplicate existing records on unique key values. If you specify
REPLACE
, new rows replace existing rows that have the same unique key
value. If you specify IGNORE
, input rows that duplicate an existing
row on a unique key value are skipped. If you don't specify either option, an
error occurs when a duplicate key value is found, and the rest of the text
file is ignored.
If you load data from a local file using the LOCAL
keyword, the server
has no way to stop transmission of the file in the middle of the operation,
so the default bahavior is the same as if IGNORE
is specified.
LOAD DATA INFILE
is the complement of SELECT ... INTO OUTFILE
.
See section 7.12 SELECT
syntax.
To write data from a database to a file, use SELECT ... INTO OUTFILE
.
To read the file back into the database, use LOAD DATA INFILE
.
The syntax of the FIELDS
and LINES
clauses is the same for
both commands. Both clauses are optional, but FIELDS
must precede LINES
if both are specified.
If you specify a FIELDS
clause,
each of its subclauses (TERMINATED BY
, [OPTIONALLY] ENCLOSED
BY
and ESCAPED BY
) is also optional, except that you must
specify at least one of them.
If you don't specify a FIELDS
clause, the defaults are the
same as if you had written this:
FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'
If you don't specify a LINES
clause, the default
is the same as if you had written this:
LINES TERMINATED BY '\n'
In other words, the defaults cause LOAD DATA INFILE
to act as follows
when reading input:
Conversely, the defaults cause SELECT ... INTO OUTFILE
to act as
follows when writing output:
Note that to write FIELDS ESCAPED BY '\\'
, you must specify two
backslashes for the value to be read as a single backslash.
The IGNORE number LINES
option can be used to ignore a header of
column names at the start of the file:
mysql> LOAD DATA INFILE "/tmp/file_name" into table test IGNORE 1 LINES;
When you use SELECT ... INTO OUTFILE
in tandem with LOAD
DATA INFILE
to write data from a database into a file and then read
the file back into the database later, the field and line handling
options for both commands must match. Otherwise, LOAD DATA
INFILE
will not interpret the contents of the file properly. Suppose
you use SELECT ... INTO OUTFILE
to write a file with
fields delimited by commas:
mysql> SELECT * FROM table1 INTO OUTFILE 'data.txt' FIELDS TERMINATED BY ',' FROM ...
To read the comma-delimited file back in, the correct statement would be:
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2 FIELDS TERMINATED BY ',';
If instead you tried to read in the file with the statement shown below, it
wouldn't work because it instructs LOAD DATA INFILE
to look for
tabs between fields:
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2 FIELDS TERMINATED BY '\t';
The likely result is that each input line would be interpreted as a single field.
LOAD DATA INFILE
can be used to read files obtained from
external sources, too. For example, a file in dBASE format will have
fields separated by commas and enclosed in double quotes. If lines in
the file are terminated by newlines, the command shown below
illustrates the field and line handling options you would use to load
the file:
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name FIELDS TERMINATED BY ',' ENCLOSED BY '"' LINES TERMINATED BY '\n';
Any of the field or line handling options may specify an empty string
(''
). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY
and FIELDS ESCAPED BY
values must be a single character. The
FIELDS TERMINATED BY
and LINES TERMINATED BY
values may
be more than one character. For example, to write lines that are
terminated by carriage return-linefeed pairs, or to read a file
containing such lines, specify a LINES TERMINATED BY '\r\n'
clause.
FIELDS [OPTIONALLY] ENCLOSED BY
controls quoting of fields. For
output (SELECT ... INTO OUTFILE
), if you omit the word
OPTIONALLY
, all fields are enclosed by the ENCLOSED BY
character. An example of such output (using a comma as the field
delimiter) is shown below:
"1","a string","100.20" "2","a string containing a , comma","102.20" "3","a string containing a \" quote","102.20" "4","a string containing a \", quote and comma","102.20"
If you specify OPTIONALLY
, the ENCLOSED BY
character is
used only to enclose CHAR
and VARCHAR
fields:
1,"a string",100.20 2,"a string containing a , comma",102.20 3,"a string containing a \" quote",102.20 4,"a string containing a \", quote and comma",102.20
Note that occurrences of the ENCLOSED BY
character within a
field value are escaped by prefixing them with the ESCAPED BY
character. Also note that if you specify an empty ESCAPED BY
value, it is possible to generate output that cannot be read properly by
LOAD DATA INFILE
. For example, the output just shown above would
appear as shown below if the escape character is empty. Observe that the
second field in the fourth line contains a comma following the quote, which
(erroneously) appears to terminate the field:
1,"a string",100.20 2,"a string containing a , comma",102.20 3,"a string containing a " quote",102.20 4,"a string containing a ", quote and comma",102.20
For input, the ENCLOSED BY
character, if present, is stripped from the
ends of field values. (This is true whether or not OPTIONALLY
is
specified; OPTIONALLY
has no effect on input interpretation.)
Occurrences of the ENCLOSED BY
character preceded by the
ESCAPED BY
character are interpreted as part of the current field
value. In addition, duplicated ENCLOSED BY
characters occurring
within fields are interpreted as single ENCLOSED BY
characters if the
field itself starts with that character. For example, if ENCLOSED BY
'"'
is specified, quotes are handled as shown below:
"The ""BIG"" boss" -> The "BIG" boss The "BIG" boss -> The "BIG" boss The ""BIG"" boss -> The ""BIG"" boss
FIELDS ESCAPED BY
controls how to write or read special characters.
If the FIELDS ESCAPED BY
character is not empty, it is used to prefix
the following characters on output:
FIELDS ESCAPED BY
character
FIELDS [OPTIONALLY] ENCLOSED BY
character
FIELDS TERMINATED BY
and
LINES TERMINATED BY
values
0
(what is actually written following the escape character is
ASCII '0'
, not a zero-valued byte)
If the FIELDS ESCAPED BY
character is empty, no characters are escaped.
It is probably not a good idea to specify an empty escape character,
particularly if field values in your data contain any of the characters in
the list just given.
For input, if the FIELDS ESCAPED BY
character is not empty, occurrences
of that character are stripped and the following character is taken literally
as part of a field value. The exceptions are an escaped `0' or
`N' (e.g., \0
or \N
if the escape character is
`\'). These sequences are interpreted as ASCII 0
(a zero-valued
byte) and NULL
. See below for the rules on NULL
handling.
For more information about `\'-escape syntax, see section 7.1 Literals: how to write strings and numbers.
In certain cases, field and line handling options interact:
LINES TERMINATED BY
is an empty string and FIELDS
TERMINATED BY
is non-empty, lines are also terminated with
FIELDS TERMINATED BY
.
FIELDS TERMINATED BY
and FIELDS ENCLOSED BY
values are
both empty (''
), a fixed-row (non-delimited) format is used. With
fixed-row format, no delimiters are used between fields. Instead, column
values are written and read using the ``display'' widths of the columns. For
example, if a column is declared as INT(7)
, values for the column are
written using 7-character fields. On input, values for the column are
obtained by reading 7 characters. Fixed-row format also affects handling of
NULL
values; see below. Note that fixed size format will not work
if you are using a multi-byte character set.
Handling of NULL
values varies, depending on the FIELDS
and
LINES
options you use:
FIELDS
and LINES
values,
NULL
is written as \N
for output and \N
is read
as NULL
for input (assuming the ESCAPED BY
character
is `\').
FIELDS ENCLOSED BY
is not empty, a field containing the literal
word NULL
as its value is read as a NULL
value (this differs
from the word NULL
enclosed within FIELDS ENCLOSED BY
characters, which is read as the string 'NULL'
).
FIELDS ESCAPED BY
is empty, NULL
is written as the word
NULL
.
FIELDS TERMINATED BY
and
FIELDS ENCLOSED BY
are both empty), NULL
is written as an empty
string. Note that this causes both NULL
values and empty strings in
the table to be indistinguishable when written to the file since they are
both written as empty strings. If you need to be able to tell the two apart
when reading the file back in, you should not use fixed-row format.
Some cases are not supported by LOAD DATA INFILE
:
FIELDS TERMINATED BY
and FIELDS ENCLOSED
BY
both empty) and BLOB
or TEXT
columns.
LOAD DATA INFILE
won't be able to interpret the input properly.
For example, the following FIELDS
clause would cause problems:
FIELDS TERMINATED BY '"' ENCLOSED BY '"'
FIELDS ESCAPED BY
is empty, a field value that contains an occurrence
of FIELDS ENCLOSED BY
or LINES TERMINATED BY
followed by the FIELDS TERMINATED BY
value will cause LOAD
DATA INFILE
to stop reading a field or line too early.
This happens because LOAD DATA INFILE
cannot properly determine
where the field or line value ends.
The following example loads all columns of the persondata
table:
mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;
No field list is specified, so LOAD DATA INFILE
expects input rows
to contain a field for each table column. The default FIELDS
and
LINES
values are used.
If you wish to load only some of a table's columns, specify a field list:
mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata (col1,col2,...);
You must also specify a field list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.
If a row has too few fields, the columns for which no input field is present
are set to default values. Default value assignment is described in
section 7.7 CREATE TABLE
syntax.
An empty field value is interpreted differently than if the field value is missing:
0
.
TIMESTAMP
columns are only set to the current date and time if there
is a NULL
value for the column, or (for the first TIMESTAMP
column only) if the TIMESTAMP
column is left out from the field list
when a field list is specified.
If an input row has too many fields, the extra fields are ignored and the number of warnings is incremented.
LOAD DATA INFILE
regards all input as strings, so you can't use
numeric values for ENUM
or SET
columns the way you can with
INSERT
statements. All ENUM
and SET
values must be
specified as strings!
If you are using the C API, you can get information about the query by
calling the API function mysql_info()
when the LOAD DATA INFILE
query finishes. The format of the information string is shown below:
Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
Warnings occur under the same circumstances as when values are inserted
via the INSERT
statement (see section 7.14 INSERT
syntax), except
that LOAD DATA INFILE
also generates warnings when there are too few
or too many fields in the input row. The warnings are not stored anywhere;
the number of warnings can only be used as an indication if everything went
well. If you get warnings and want to know exactly why you got them, one way
to do this is to use SELECT ... INTO OUTFILE
into another file and
compare this to your original input file.
For more information about the efficiency of INSERT
versus
LOAD DATA INFILE
and speeding up LOAD DATA INFILE
,
See section 10.5.6 Speed of INSERT
querys.
UPDATE
syntaxUPDATE [LOW_PRIORITY] tbl_name SET col_name1=expr1,col_name2=expr2,... [WHERE where_definition] [LIMIT #]
UPDATE
updates columns in existing table rows with new values. The
SET
clause indicates which columns to modify and the values they
should be given. The WHERE
clause, if given, specifies which rows
should be updated. Otherwise all rows are updated.
If you specify the keyword LOW_PRIORITY
, execution of the
UPDATE
is delayed until no other clients are reading from the table.
If you access a column from tbl_name
in an expression, UPDATE
uses
the current value of the column. For example, the following statement sets
the age
column to one more than its current value:
mysql> UPDATE persondata SET age=age+1;
UPDATE
assignments are evaluated from left to right. For example, the
following statement doubles the age
column, then increments it:
mysql> UPDATE persondata SET age=age*2, age=age+1;
If you set a column to the value it currently has, MySQL notices this and doesn't update it.
UPDATE
returns the number of rows that were actually changed.
In MySQL 3.22 or later, the C API function mysql_info()
returns the number of rows that were matched and updated and the number of
warnings that occurred during the UPDATE
.
In MySQL 3.23 you can use LIMIT #
to ensure that only a given
number of rows are changed.
USE
syntaxUSE db_name
The USE db_name
statement tells MySQL to use the db_name
database as the default database for subsequent queries. The database remains
current until the end of the session, or until another USE
statement
is issued:
mysql> USE db1; mysql> SELECT count(*) FROM mytable; # selects from db1.mytable mysql> USE db2; mysql> SELECT count(*) FROM mytable; # selects from db2.mytable
Making a particular database current by means of the USE
statement
does not preclude you from accessing tables in other databases. The example
below accesses the author
table from the db1
database and the
editor
table from the db2
database:
mysql> USE db1; mysql> SELECT author_name,editor_name FROM author,db2.editor WHERE author.editor_id = db2.editor.editor_id;
The USE
statement is provided for Sybase compatibility.
FLUSH
syntax (clearing caches)FLUSH flush_option [,flush_option]
You should use the FLUSH
command if you want to clear some of the
internal caches MySQL uses. To execute FLUSH
, you must have
the reload privilege.
flush_option
can be any of the following:
HOSTS | Empties the host cache tables. You should flush the
host tables if some of your hosts change IP number or if you get the
error message Host ... is blocked . When more than
max_connect_errors errors occur in a row for a given host while
connection to the MySQL server, MySQL assumes
something is wrong and blocks the host from further connection requests.
Flushing the host tables allows the host to attempt to connect
again. See section 18.2.3 Host '...' is blocked error.) You can start mysqld with
-O max_connection_errors=999999999 to avoid this error message.
|
LOGS | Closes and reopens the standard and update log files. If you have specified the update log file without an extension, the extension number of the new update log file will be incremented by one relative to the previous file. |
PRIVILEGES | Reloads the privileges from the grant tables in
the mysql database.
|
TABLES | Closes all open tables. |
STATUS | Resets most status variables to zero. |
You can also access each of the commands shown above with the mysqladmin
utility, using the flush-hosts
, flush-logs
, reload
or flush-tables
commands.
KILL
syntaxKILL thread_id
Each connection to mysqld
runs in a separate thread. You can see
which threads are running with the SHOW PROCESSLIST
command, and kill
a thread with the KILL thread_id
command.
If you have the process privilege, you can see and kill all threads. Otherwise, you can see and kill only your own threads.
You can also use the mysqladmin processlist
and mysqladmin kill
commands to examine and kill threads.
SHOW
syntax (Get information about tables, columns,...)SHOW DATABASES [LIKE wild] or SHOW TABLES [FROM db_name] [LIKE wild] or SHOW COLUMNS FROM tbl_name [FROM db_name] [LIKE wild] or SHOW INDEX FROM tbl_name [FROM db_name] or SHOW STATUS or SHOW VARIABLES [LIKE wild] or SHOW PROCESSLIST or SHOW TABLE STATUS [FROM db_name] [LIKE wild] or SHOW GRANTS FOR user
SHOW
provides information about databases, tables, columns or the
server. If the LIKE wild
part is used, the wild
string can be a
string that uses the SQL `%' and `_' wildcard characters.
You can use db_name.tbl_name
as an alternative to the tbl_name
FROM db_name
syntax. These two statements are equivalent:
mysql> SHOW INDEX FROM mytable FROM mydb; mysql> SHOW INDEX FROM mydb.mytable;
SHOW DATABASES
lists the databases on the MySQL server
host. You can also get this list using the mysqlshow
command.
SHOW TABLES
lists the tables in a given database. You can also
get this list using the mysqlshow db_name
command.
Note: If a user doesn't have any privileges for a table, the table
will not show up in the output from SHOW TABLES
or mysqlshow
db_name
.
SHOW COLUMNS
lists the columns in a given table.
If the column types are different than you expect them to be based on a
CREATE TABLE
statement, note that MySQL sometimes changes column
types.
See section 7.7.1 Silent column specification changes.
The DESCRIBE
statement provides information similar to
SHOW COLUMNS
.
See section 7.23 DESCRIBE
syntax (Get information about columns).
SHOW TABLE STATUS
(new in version 3.23) works likes SHOW
STATUS
, but provides a lot of information about each table. You can
also get this list using the mysqlshow --status db_name
command.
The following columns are returned:
Column | Meaning |
Name | Name of the table |
Type | Type of table (ISAM, MyISAM or HEAP) |
Row_format | The row storage format (Fixed, Dynamic, or Compressed) |
Rows | Number of rows |
Avg_row_length | Average row length |
Data_length | Length of the data file |
Max_data_length | Max length of the data file |
Index_length | Length of the index file |
Data_free | Number of allocated but not used bytes |
Auto_increment | Next autoincrement value |
Create_time | When the table was created |
Update_time | When the data file was last updated |
Check_time | When one last run a check on the table |
Create_options | Extra options used with CREATE TABLE
|
Comment | The comment used when creating the table (or some information why MySQL couldn't access the table information). |
SHOW FIELDS
is a synonym for SHOW COLUMNS
and
SHOW KEYS
is a synonym for SHOW INDEX
. You can also
list a table's columns or indexes with mysqlshow db_name tbl_name
or mysqlshow -k db_name tbl_name
.
SHOW INDEX
returns the index information in a format that closely
resembles the SQLStatistics
call in ODBC. The following columns
are returned:
Column | Meaning |
Table | Name of the table |
Non_unique | 0 if the index can't contain duplicates. |
Key_name | Name of the index |
Seq_in_index | Column sequence number in index, starting with 1. |
Column_name | Column name. |
Collation | How the column is sorted in the index. In
MySQL, this can have values A (Ascending) or NULL
(Not sorted).
|
Cardinality | Number of unique values in the index.
This is updated by running isamchk -a .
|
Sub_part | Number of indexed characters if the column is
only partly indexed. NULL if the entire key is indexed.
|
SHOW STATUS
provides server status information
(like mysqladmin extended-status
). The output resembles that shown
below, though the format and numbers may differ somewhat:
+--------------------------+--------+ | Variable_name | Value | +--------------------------+--------+ | Aborted_clients | 0 | | Aborted_connects | 0 | | Connections | 17 | | Created_tmp_tables | 0 | | Delayed_insert_threads | 0 | | Delayed_writes | 0 | | Delayed_errors | 0 | | Flush_commands | 2 | | Handler_delete | 2 | | Handler_read_first | 0 | | Handler_read_key | 1 | | Handler_read_next | 0 | | Handler_read_rnd | 35 | | Handler_update | 0 | | Handler_write | 2 | | Key_blocks_used | 0 | | Key_read_requests | 0 | | Key_reads | 0 | | Key_write_requests | 0 | | Key_writes | 0 | | Max_used_connections | 1 | | Not_flushed_key_blocks | 0 | | Not_flushed_delayed_rows | 0 | | Open_tables | 1 | | Open_files | 2 | | Open_streams | 0 | | Opened_tables | 11 | | Questions | 14 | | Slow_queries | 0 | | Threads_connected | 1 | | Threads_running | 1 | | Uptime | 149111 | +--------------------------+--------+
The status variables listed above have the following meaning:
Aborted_clients | Number of connections that has been aborted because the client has died without closing the connection properly. |
Aborted_connects | Number of tries to connect to the MySQL server that has failed. |
Connections | Number of connection attempts to the MySQL server. |
Created_tmp_tables | Number of implicit temporary tables that has been created while executing statements. |
Delayed_insert_threads | Number of delayed insert handler threads in use. |
Delayed_writes | Number of rows written with INSERT DELAYED .
|
Delayed_errors | Number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key ).
|
Flush_commands | Number of executed FLUSH commands.
|
Handler_delete | Number of requests to delete a row from a table. |
Handler_read_first | Number of requests to read the first row in a table. |
Handler_read_key | Number of requests to read a row based on a key. |
Handler_read_next | Number of requests to read next row in key order. |
Handler_read_rnd | Number of requests to read a row based on a fixed position. |
Handler_update | Number of requests to update a row in a table. |
Handler_write | Number of requests to insert a row in a table. |
Key_blocks_used | The number of used blocks in the key cache. |
Key_read_requests | The number of requests to read a key block from the cache. |
Key_reads | The number of physical reads of a key block from disk. |
Key_write_requests | The number of requests to write a key block to the cache. |
Key_writes | The number of physical writes of a key block to disk. |
Max_used_connections | The maximum number of connections that has been in use simultaneously. |
Not_flushed_key_blocks | Keys blocks in the key cache that has changed but hasn't yet been flushed to disk. |
Not_flushed_delayed_rows | Number of rows waiting to be written in INSERT DELAY queues.
|
Open_tables | Number of tables that are open. |
Open_files | Number of files that are open. |
Open_streams | Number of streams that are open (used mainly for logging) |
Opened_tables | Number of tables that has been opened. |
Questions | Number of queries sent to the server. |
Slow_queries | Number of queries that has taken more than long_query_time
|
Threads_connected | Number of currently open connections. |
Threads_running | Number of threads that are not sleeping. |
Uptime | How many seconds the server has been up. |
Some comments about the above:
Opened_tables
is big, then your table_cache
variable is probably too small.
key_reads
is big, then your key_cache
is probably too
small. The cache hit rate can be calculated with
key_reads
/key_read_requests
.
Handler_read_rnd
is big, then you have a probably a lot of queries
that requires MySQL to scan whole tables or you have joins that doesn't use
keys properly.
SHOW VARIABLES
shows the values of the some of MySQL system
variables. You can also get this information using the mysqladmin
variables
command. If the default values are unsuitable, you can set most
of these variables using command-line options when mysqld
starts up.
The output resembles that shown below, though the format and numbers may
differ somewhat:
+------------------------+--------------------------+ | Variable_name | Value | +------------------------+--------------------------+ | back_log | 5 | | connect_timeout | 5 | | basedir | /my/monty/ | | datadir | /my/monty/data/ | | delayed_insert_limit | 100 | | delayed_insert_timeout | 300 | | delayed_queue_size | 1000 | | join_buffer_size | 131072 | | flush_time | 0 | | key_buffer_size | 1048540 | | language | /my/monty/share/english/ | | log | OFF | | log_update | OFF | | long_query_time | 10 | | low_priority_updates | OFF | | max_allowed_packet | 1048576 | | max_connections | 100 | | max_connect_errors | 10 | | max_delayed_threads | 20 | | max_heap_table_size | 16777216 | | max_join_size | 4294967295 | | max_sort_length | 1024 | | max_tmp_tables | 32 | | net_buffer_length | 16384 | | port | 3306 | | protocol-version | 10 | | record_buffer | 131072 | | skip_locking | ON | | socket | /tmp/mysql.sock | | sort_buffer | 2097116 | | table_cache | 64 | | thread_stack | 131072 | | tmp_table_size | 1048576 | | tmpdir | /machine/tmp/ | | version | 3.23.0-alpha-debug | | wait_timeout | 28800 | +------------------------+--------------------------+
See section 10.2.3 Tuning server parameters.
SHOW PROCESSLIST
shows you which threads are running. You can also
get this information using the mysqladmin processlist
command. If you
have the process privilege, you can see all threads. Otherwise, you
can see only your own threads.
See section 7.20 KILL
syntax.
SHOW GRANTS FOR user
lists the grant commands that must be issued to
duplicate the grants for a user.
mysql> SHOW GRANTS FOR root@localhost; +---------------------------------------------------------------------+ | Grants for root@localhost | +---------------------------------------------------------------------+ | GRANT ALL PRIVILEGES ON *.* TO 'root''localhost' WITH GRANT OPTION | +---------------------------------------------------------------------+
EXPLAIN
syntax (Get information about a SELECT
)EXPLAIN tbl_name or EXPLAIN SELECT select_options
EXPLAIN tbl_name
is a synonym for DESCRIBE tbl_name
or
SHOW COLUMNS FROM tbl_name
.
When you precede a SELECT
statement with the keyword EXPLAIN
,
MySQL explains how it would process the SELECT
, providing
information about how tables are joined and in which order.
With the help of EXPLAIN
, you can see when you must add indexes
to tables to get a faster SELECT
that uses indexes to find the
records. You can also see if the optimizer joins the tables in an optimal
order. To force the optimizer to use a specific join order for a
SELECT
statement, add a STRAIGHT_JOIN
clause.
For non-simple joins, EXPLAIN
returns a row of information for each
table used in the SELECT
statement. The tables are listed in the order
they would be read. MySQL resolves all joins using a single-sweep
multi-join method. This means that MySQL reads a row from the first
table, then finds a matching row in the second table, then in the third table
and so on. When all tables are processed, it outputs the selected columns and
backtracks through the table list until a table is found for which there are
more matching rows. The next row is read from this table and the process
continues with the next table.
Output from EXPLAIN
includes the following columns:
table
type
possible_keys
possible_keys
column indicates which indexes MySQL
could use to find the rows in this table. Note that this colums is
totally indepentent on the order of the tables. That means that some of
the keys in possible_keys may not the usable in practice with the
generated table order.
If this column is empty, there are no relevant indexes. In this case,
you may be able to improve the performance of your query by examining
the WHERE
clause to see if it refers to some column or columns
that would be suitable for indexing. If so, create an appropriate index
and check the query with EXPLAIN
again. See section 7.8 ALTER TABLE
syntax.
To see what indexes a table has, use SHOW INDEX FROM tbl_name
.
key
key
column indicates the key that MySQL actually
decided to use. The key is NULL
if no index was chosen.
key_len
key_len
column indicates the length of the key that
MySQL decided to use. The length is NULL
if the
key
is NULL
. Note that this tell us how many parts of a
multi part key MySQL will actually use.
ref
ref
column shows which columns or constants are used with the
key
to select rows from the table.
rows
rows
column indicates the number of rows MySQL
believe it must examine to execute the query.
Extra
Extra
column includes the text Only index
, this
means that information is retrieved from the table using only
information in the index tree. Normally, this is much faster than
scanning the entire table.
If the Extra
column includes the text where used
, it means
that a WHERE
clause will be used to restrict which rows will be
matched against the next table or sent to the client.
The different join types are listed below, ordered from best to worst type:
system
const
join type.
const
const
tables are very fast as they are read only once!
eq_ref
const
types. It is used when all parts of an index are used by
the join and the index is UNIQUE
or a PRIMARY KEY
.
ref
ref
is used if the join
uses only a leftmost prefix of the key, or if the key is not UNIQUE
or a PRIMARY KEY
(in other words, if the join cannot select a single
row based on the key value). If the key that is used matches only a few rows,
this join type is good.
range
ref
column indicates which index is used.
index
ALL
, except that only the index tree is
scanned. This is usually faster than ALL
, as the index file is usually
smaller than the data file.
ALL
const
, and usually very bad in all other
cases. You normally can avoid ALL
by adding more indexes, so that
the row can be retrieved based on constant values or column values from
earlier tables.
You can get a good indication of how good a join is by multiplying all values
in the rows
column of the EXPLAIN
output. This should tell you
roughly how many rows MySQL must examine to execute the query. This
number is also used when you restrict queries with the max_join_size
variable.
See section 10.2.3 Tuning server parameters.
The following example shows how a JOIN
can be optimized progressively
using the information provided by EXPLAIN
.
Suppose you have the SELECT
statement shown below, that you examine
using EXPLAIN
:
EXPLAIN SELECT tt.TicketNumber, tt.TimeIn, tt.ProjectReference, tt.EstimatedShipDate, tt.ActualShipDate, tt.ClientID, tt.ServiceCodes, tt.RepetitiveID, tt.CurrentProcess, tt.CurrentDPPerson, tt.RecordVolume, tt.DPPrinted, et.COUNTRY, et_1.COUNTRY, do.CUSTNAME FROM tt, et, et AS et_1, do WHERE tt.SubmitTime IS NULL AND tt.ActualPC = et.EMPLOYID AND tt.AssignedPC = et_1.EMPLOYID AND tt.ClientID = do.CUSTNMBR;
For this example, assume that:
Table | Column | Column type |
tt | ActualPC | CHAR(10)
|
tt | AssignedPC | CHAR(10)
|
tt | ClientID | CHAR(10)
|
et | EMPLOYID | CHAR(15)
|
do | CUSTNMBR | CHAR(15)
|
Table | Index |
tt | ActualPC
|
tt | AssignedPC
|
tt | ClientID
|
et | EMPLOYID (primary key)
|
do | CUSTNMBR (primary key)
|
tt.ActualPC
values aren't evenly distributed.
Initially, before any optimizations have been performed, the EXPLAIN
statement produces the following information:
table type possible_keys key key_len ref rows Extra et ALL PRIMARY NULL NULL NULL 74 do ALL PRIMARY NULL NULL NULL 2135 et_1 ALL PRIMARY NULL NULL NULL 74 tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 range checked for each record (key map: 35)
Since type
is ALL
for each table, this output indicates that
MySQL is doing a full join for all tables! This will take quite a
long time, as the product of the number of rows in each table must be
examined! For the case at hand, this is 74 * 2135 * 74 * 3872 =
45,268,558,720
rows. If the tables were bigger, you can only imagine how
long it would take...
One problem here is that MySQL can't (yet) use indexes on columns
efficiently if they are declared differently. In this context,
VARCHAR
and CHAR
are the same unless they are declared as
different lengths. Since tt.ActualPC
is declared as CHAR(10)
and et.EMPLOYID
is declared as CHAR(15)
, there is a length
mismatch.
To fix this disparity between column lengths, use ALTER TABLE
to
lengthen ActualPC
from 10 characters to 15 characters:
mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);
Now tt.ActualPC
and et.EMPLOYID
are both VARCHAR(15)
.
Executing the EXPLAIN
statement again produces this result:
table type possible_keys key key_len ref rows Extra tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 where used do ALL PRIMARY NULL NULL NULL 2135 range checked for each record (key map: 1) et_1 ALL PRIMARY NULL NULL NULL 74 range checked for each record (key map: 1) et eq_ref PRIMARY PRIMARY 15 tt.ActualPC 1
This is not perfect, but is much better (the product of the rows
values is now less by a factor of 74). This version is executed in a couple
of seconds.
A second alteration can be made to eliminate the column length mismatches
for the tt.AssignedPC = et_1.EMPLOYID
and tt.ClientID =
do.CUSTNMBR
comparisons:
mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15), MODIFY ClientID VARCHAR(15);
Now EXPLAIN
produces the output shown below:
table type possible_keys key key_len ref rows Extra et ALL PRIMARY NULL NULL NULL 74 tt ref AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used et_1 eq_ref PRIMARY PRIMARY 15 tt.AssignedPC 1 do eq_ref PRIMARY PRIMARY 15 tt.ClientID 1
This is ``almost'' as good as it can get.
The remaining problem is that, by default, MySQL assumes that values
in the tt.ActualPC
column are evenly distributed, and that isn't the
case for the tt
table. Fortunately, it is easy to tell MySQL
about this:
shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt shell> mysqladmin refresh
Now the join is ``perfect'', and EXPLAIN
produces this result:
table type possible_keys key key_len ref rows Extra tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 where used et eq_ref PRIMARY PRIMARY 15 tt.ActualPC 1 et_1 eq_ref PRIMARY PRIMARY 15 tt.AssignedPC 1 do eq_ref PRIMARY PRIMARY 15 tt.ClientID 1
Note that the rows
column in the output from EXPLAIN
is an
``educated guess'' from the MySQL join optimizer; To optimize a
query, you should check if the numbers are even close to the truth. If not,
you may get better performance by using STRAIGHT_JOIN
in your
SELECT
statement and trying to list the tables in a different order in
the FROM
clause.
DESCRIBE
syntax (Get information about columns){DESCRIBE | DESC} tbl_name {col_name | wild}
DESCRIBE
provides information about a table's columns. col_name
may be a column name or a string containing the SQL `%' and `_'
wildcard characters.
If the column types are different than you expect them to be based on a
CREATE TABLE
statement, note that MySQL sometimes changes column
types.
See section 7.7.1 Silent column specification changes.
This statement is provided for Oracle compatibility.
The SHOW
statement provides similar information.
See section 7.21 SHOW
syntax (Get information about tables, columns,...).
LOCK TABLES/UNLOCK TABLES
syntaxLOCK TABLES tbl_name [AS alias] {READ | [LOW_PRIORITY] WRITE} [, tbl_name {READ | [LOW_PRIORITY] WRITE} ...] ... UNLOCK TABLES
LOCK TABLES
locks tables for the current thread. UNLOCK TABLES
releases any locks held by the current thread. All tables that are locked by
the current thread are automatically unlocked when the thread issues another
LOCK TABLES
, or when the connection to the server is closed.
If a thread obtains a READ
lock on a table, that thread (and all other
threads) can only read from the table. If a thread obtains a WRITE
lock on a table, then only the thread holding the lock can READ
from
or WRITE
to the table. Other threads are blocked.
Each thread waits (without timing out) until it obtains all the locks it has requested.
WRITE
locks normally have higher priority than READ
locks, to
ensure that updates are processed as soon as possible. This means that if one
thread obtains a READ
lock and then another thread requests a
WRITE
lock, subsequent READ
lock requests will wait until the
WRITE
thread has gotten the lock and released it. You can use
LOW_PRIORITY WRITE
locks to allow other threads to obtain READ
locks while the thread is waiting for the WRITE
lock. You should only
use LOW_PRIORITY WRITE
locks if you are sure that there will
eventually be a time when no threads will have a READ
lock.
When you use LOCK TABLES
, you must lock all tables that you are
going to use! If you are using a table multiple times in a query (with
aliases), you must get a lock for each alias! This policy ensures that
table locking is deadlock free.
Note that you should NOT lock any tables that you are using with
INSERT DELAYED
. This is because that in this case the INSERT
is done by a separate thread.
Normally, you don't have to lock tables, as all single UPDATE
statements
are atomic; no other thread can interfere with any other currently executing
SQL statement. There are a few cases when you would like to lock tables
anyway:
READ
-locked table and no other
thread can read a WRITE
-locked table.
LOCK TABLES
if you want to ensure that no other thread comes between a
SELECT
and an UPDATE
. The example shown below
requires LOCK TABLES
in order to execute safely:
mysql> LOCK TABLES trans READ, customer WRITE; mysql> select sum(value) from trans where customer_id= some_id; mysql> update customer set total_value=sum_from_previous_statement where customer_id=some_id; mysql> UNLOCK TABLES;Without
LOCK TABLES
, there is a chance that another thread might
insert a new row in the trans
table between execution of the
SELECT
and UPDATE
statements.
By using incremental updates (UPDATE customer SET
value=value+new_value
) or the LAST_INSERT_ID()
function, you can
avoid using LOCK TABLES
in many cases.
You can also solve some cases by using the user-level lock functions
GET_LOCK()
and RELEASE_LOCK()
. These locks are saved in a hash
table in the server and implemented with pthread_mutex_lock()
and
pthread_mutex_unlock()
for high speed.
See section 7.4.12 Miscellaneous functions.
See section 10.2.8 How MySQL locks tables, for more information on locking policy.
SET OPTION
syntaxSET [OPTION] SQL_VALUE_OPTION= value, ...
SET OPTION
sets various options that affect the operation of the
server or your client. Any option you set remains in effect until the
current session ends, or until you set the option to a different value.
CHARACTER SET character_set_name | DEFAULT
character_set_name
is
cp1251_koi8
, but you can easily add new mappings by editing the
`sql/convert.cc' file in the MySQL source distribution. The
default mapping can be restored by using a character_set_name
value of
DEFAULT
.
Note that the syntax for setting the CHARACTER SET
option differs
from the syntax for setting the other options.
PASSWORD = PASSWORD('some password')
PASSWORD FOR user = PASSWORD('some password')
mysql
database can do this. The user should be
given in user@hostname
format, where user
and hostname
are exactly as they are listed in the User
and Host
columns of
the mysql.user
table entry. For example, if you had an entry with
User
and Host
fields of 'bob'
and '%.loc.gov'
,
you would write:
mysql> SET PASSWORD FOR bob@"%.loc.gov" = PASSWORD("newpass"); or mysql> UPDATE mysql.user SET password=PASSWORD("newpass") where user="bob' and host="%.loc.gov";
SQL_AUTO_IS_NULL = 0 | 1
1
(default) then one can find the last inserted row
for a table with an auto_increment row with the following construct:
WHERE auto_increment_column IS NULL
. This is used by some
ODBC programs like Access.
SQL_BIG_TABLES = 0 | 1
1
, all temporary tables are stored on disk rather than in
memory. This will be a little slower, but you will not get the error
The table tbl_name is full
for big SELECT
operations that
require a large temporary table. The default value for a new connection is
0
(i.e., use in-memory temporary tables).
SQL_BIG_SELECTS = 0 | 1
0
, MySQL will abort if a SELECT
is attempted
that probably will take a very long time. This is useful when an inadvisable
WHERE
statement has been issued. A big query is defined as a
SELECT
that probably will have to examine more than
max_join_size
rows. The default value for a new connection is
1
(which will allow all SELECT
statements).
SQL_LOW_PRIORITY_UPDATES = 0 | 1
1
, all INSERT
, UPDATE
, DELETE
and
and LOCK TABLE WRITE
statements wait until there is no pending
SELECT
or LOCK TABLE READ
on the affected table.
SQL_SELECT_LIMIT = value | DEFAULT
SELECT
statements. If
a SELECT
has a LIMIT
clause, the LIMIT
takes precedence
over the value of SQL_SELECT_LIMIT
. The default value for a new
connection is ``unlimited''. If you have changed the limit, the default value
can be restored by using a SQL_SELECT_LIMIT
value of DEFAULT
.
SQL_LOG_OFF = 0 | 1
1
, no logging will be done to the standard log for this
client, if the client has the process privilege. This does not
affect the update log!
SQL_LOG_UPDATE = 0 | 1
0
, no logging will be done to the update log for the client,
if the client has the process privilege. This does not affect the
standard log!
TIMESTAMP = timestamp_value | DEFAULT
LAST_INSERT_ID = #
LAST_INSERT_ID()
. This is stored in
the update log when you use LAST_INSERT_ID()
in a command that updates
a table.
INSERT_ID = #
INSERT
command when inserting
an AUTO_INCREMENT
value. This is mainly used with the update log.
GRANT
and REVOKE
syntaxGRANT priv_type [(column_list)] [, priv_type [(column_list)] ...] ON {tbl_name | * | *.* | db_name.*} TO user_name [IDENTIFIED BY 'password'] [, user_name [IDENTIFIED BY 'password'] ...] [WITH GRANT OPTION] REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...] ON {tbl_name | * | *.* | db_name.*} FROM user_name [, user_name ...]
GRANT
is implemented in MySQL 3.22.11 or later. For
earlier MySQL versions, the GRANT
statement does nothing.
The GRANT
and REVOKE
commands allow system administrators to
grant and revoke rights to MySQL users at four privilege levels:
mysql.user
table.
mysql.db
and mysql.host
tables.
mysql.tables_priv
table.
mysql.columns_priv
table.
For examples of how GRANT
works, see section 6.11 Adding new user privileges to MySQL.
For the GRANT
and REVOKE
statements, priv_type
may be
specified as any of the following:
ALL PRIVILEGES FILE RELOAD ALTER INDEX SELECT CREATE INSERT SHUTDOWN DELETE PROCESS UPDATE DROP REFERENCES USAGE
ALL
is a synonym for ALL PRIVILEGES
. REFERENCES
is not
yet implemented. USAGE
is currently a synonym for ``no privileges''.
It can be used when you want to create a user that has no privileges.
To revoke the grant privilege from a user, use a priv_type
value of GRANT OPTION
:
REVOKE GRANT OPTION ON ... FROM ...;
The only priv_type
values you can specify for a table are SELECT
,
INSERT
, UPDATE
, DELETE
, CREATE
, DROP
,
GRANT
, INDEX
and ALTER
.
The only priv_type
values you can specify for a column (that is, when
you use a column_list
clause) are SELECT
, INSERT
and
UPDATE
.
You can set global privileges by using ON *.*
syntax. You can set
database privileges by using ON db_name.*
syntax. If you specify
ON *
and you have a current database, you will set the privileges for
that database. (Warning: If you specify ON *
and you
don't have a current database, you will affect the global privileges!)
In order to accommodate granting rights to users from arbitrary hosts,
MySQL supports specifying the user_name
value in the form
user@host
. If you want to specify a user
string
containing special characters (such as `-'), or a host
string
containing special characters or wildcard characters (such as `%'), you
can quote the user or host name (e.g., 'test-user'@'test-hostname'
).
You can specify wildcards in the hostname. For example,
user@"%.loc.gov"
applies to user
for any host in the
loc.gov
domain, and user@"144.155.166.%"
applies to user
for any host in the 144.155.166
class C subnet.
The simple form user
is a synonym for user@"%"
.
Note: If you allow anonymous users to connect to the MySQL
server (which is the default), you should also add all local users as
user@localhost
because otherwise the anonymous user entry for the
local host in the mysql.user
table will be used when the user tries to
log into the MySQL server from the local machine! Anonymous users
are defined by inserting entries with User=''
into the
mysql.user
table. You can verify if this applies to you by executing
this query:
mysql> SELECT Host,User FROM mysql.user WHERE User='';
For the moment, GRANT
only supports host, table, database and
column names up to 60 characters long. A user name can be up to 16
characters.
The privileges for a table or column are formed from the
logical OR of the privileges at each of the four privilege
levels. For example, if the mysql.user
table specifies that a
user has a global select privilege, this can't be denied by an
entry at the database, table or column level.
The privileges for a column can be calculated as follows:
global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges
In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. :) The details of the privilege-checking procedure are presented in section 6 The MySQL access privilege system.
If you grant privileges for a user/hostname combination that does not exist
in the mysql.user
table, an entry is added and remains there until
deleted with a DELETE
command. In other words, GRANT
may
create user
table entries, but REVOKE
will not remove them;
you must do that explicitly using DELETE
.
In MySQL 3.22.12 or later,
if a new user is created or if you have global grant privileges, the user's
password will be set to the password specified by the IDENTIFIED BY
clause, if one is given. If the user already had a password, it is replaced
by the new one.
Warning: If you create a new user but do not specify an
IDENTIFIED BY
clause, the user has no password. This is insecure.
Passwords can also be set with the SET PASSWORD
command.
See section 7.25 SET OPTION
syntax.
If you grant privileges for a database, an entry in the mysql.db
table is created if needed. When all privileges for the database have been
removed with REVOKE
, this entry is deleted.
If a user doesn't have any privileges on a table, the table is not displayed
when the user requests a list of tables (e.g., with a SHOW TABLES
statement).
The WITH GRANT OPTION
clause gives the user the ability to give
to other users any privileges the user has at the specified privilege level.
You should be careful to whom you give the grant privilege, as two
users with different privileges may be able to join privileges!
You cannot grant another user a privilege you don't have yourself; the grant privilege allows you to give away only those privileges you possess.
Be aware that when you grant a user the grant privilege at a
particular privilege level, any privileges the user already possesses (or
is given in the future!) at that level are also grantable by that user.
Suppose you grant a user the insert privilege on a database. If
you then grant the select privilege on the database and specify
WITH GRANT OPTION
, the user can give away not only the select
privilege, but also insert. If you then grant the update
privilege to the user on the database, the user can give away the
insert, select and update.
You should not grant alter privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables!
Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit.
When mysqld
starts, all privileges are read into memory.
Database, table and column privileges take effect at once and
user-level privileges take effect the next time the user connects.
Modifications to the grant tables that you perform using GRANT
or
REVOKE
are noticed by the server immediately.
If you modify the grant tables manually (using INSERT
, UPDATE
,
etc.), you should execute a FLUSH PRIVILEGES
statement or run
mysqladmin flush-privileges
to tell the server to reload the grant
tables.
See section 6.9 When privilege changes take effect.
The biggest differences between the ANSI SQL and MySQL versions of
GRANT
are:
REVOKE
commands or by manipulating the
MySQL grant tables.
CREATE INDEX
syntaxCREATE [UNIQUE] INDEX index_name ON tbl_name (col_name[(length)],... )
The CREATE INDEX
statement doesn't do anything in MySQL prior
to version 3.22. In 3.22 or later, CREATE INDEX
is mapped to an
ALTER TABLE
statement to create indexes.
See section 7.8 ALTER TABLE
syntax.
Normally, you create all indexes on a table at the time the table itself
is created with CREATE TABLE
.
See section 7.7 CREATE TABLE
syntax.
CREATE INDEX
allows you to add indexes to existing tables.
A column list of the form (col1,col2,...)
creates a multiple-column
index. Index values are formed by concatenating the values of the given
columns.
For CHAR
and VARCHAR
columns, indexes can be created that
use only part of a column, using col_name(length)
syntax. (On
BLOB
and TEXT
columns the length is required). The
statement shown below creates an index using the first 10 characters of
the name
column:
mysql> CREATE INDEX part_of_name ON customer (name(10));
Since most names usually differ in the first 10 characters, this index should
not be much slower than an index created from the entire name
column.
Also, using partial columns for indexes can make the index file much smaller,
which could save a lot of disk space and might also speed up INSERT
operations!
Note that you can only add a index on a column that can have NULL
values or on a BLOB
/TEXT
column if you are useing
MySQL version 3.23.2 or newer and are using the MyISAM
table type.
For more information about how MySQL uses indexes, see section 10.4 MySQL index use.
DROP INDEX
syntaxDROP INDEX index_name ON tbl_name
DROP INDEX
drops the index named index_name
from the table
tbl_name
. DROP INDEX
doesn't do anything in MySQL
prior to version 3.22. In 3.22 or later, DROP INDEX
is mapped to an
ALTER TABLE
statement to drop the index.
See section 7.8 ALTER TABLE
syntax.
The MySQL server supports the # to end of line
, --
to end of line
and /* in-line or multiple-line */
comment
styles:
mysql> select 1+1; # This comment continues to the end of line mysql> select 1+1; -- This comment continues to the end of line mysql> select 1 /* this is an in-line comment */ + 1; mysql> select 1+ /* this is a multiple-line comment */ 1;
Note that the --
comment style requires you to have at least one space
after the --
!
Although the server understands the comment syntax just described,
there are some limitations on the way that the mysql
client
parses /* ... */
comments:
mysql
interactively, you can tell that it
has gotten confused like this because the prompt changes from mysql>
to '>
or ">
.
These limitations apply both when you run mysql
interactively
and when you put commands in a file and tell mysql
to read its
input from that file with mysql < some-file
.
MySQL doesn't support the `--' ANSI SQL comment style. See section 5.4.7 `--' as the start of a comment.
CREATE FUNCTION/DROP FUNCTION
syntaxCREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|REAL|INTEGER} SONAME shared_library_name DROP FUNCTION function_name
A user-definable function (UDF) is a way to extend MySQL with a new
function that works like native (built in) MySQL functions such as
ABS()
and CONCAT()
.
AGGREGATE
is a new option for MySQL 3.23. An
AGGREGATE
function works exactly like a native MySQL
GROUP
function like SUM
or COUNT()
.
CREATE FUNCTION
saves the function's name, type and shared library
name in the mysql.func
system table. You must have the
insert and delete privileges for the mysql
database
to create and drop functions.
All active functions are reloaded each time the server starts, unless
you start mysqld
with the --skip-grant-tables
option. In
this case, UDF initialization is skipped and UDFs are unavailable.
(An active function is one that has been loaded with CREATE FUNCTION
and not removed with DROP FUNCTION
.)
For instructions on writing user-definable functions, see section 14 Adding new functions to MySQL. For the UDF mechanism to work, functions must be written in C or
C++, your operating system must support dynamic loading and you must have
compiled mysqld
dynamically (not static).
A common problem stems from trying to create a table with column names that
use the names of datatypes or functions built into MySQL, such as
TIMESTAMP
or GROUP
. You're allowed to do it (for example,
ABS
is an allowed column name), but whitespace is not allowed between
a function name and the `(' when using functions whose names are also
column names.
The following words are explicitly reserved in MySQL. Most of
them are forbidden by ANSI SQL92 as column and/or table names
(for example, group
).
A few are reserved because MySQL needs them and is
(currently) using a yacc
parser:
action | add | aggregate | all
|
alter | after | and | as
|
asc | avg | avg_row_length | auto_increment
|
between | bigint | bit | binary
|
blob | bool | both | by
|
cascade | case | char | character
|
change | check | checksum | column
|
columns | comment | constraint | create
|
cross | current_date | current_time | current_timestamp
|
data | database | databases | date
|
datetime | day | day_hour | day_minute
|
day_second | dayofmonth | dayofweek | dayofyear
|
dec | decimal | default | delayed
|
delay_key_write | delete | desc | describe
|
distinct | distinctrow | double | drop
|
end | else | escape | escaped
|
enclosed | enum | explain | exists
|
fields | file | first | float
|
float4 | float8 | flush | foreign
|
from | for | full | function
|
global | grant | grants | group
|
having | heap | high_priority | hour
|
hour_minute | hour_second | hosts | identified
|
ignore | in | index | infile
|
inner | insert | insert_id | int
|
integer | interval | int1 | int2
|
int3 | int4 | int8 | into
|
if | is | isam | join
|
key | keys | kill | last_insert_id
|
leading | left | length | like
|
lines | limit | load | local
|
lock | logs | long | longblob
|
longtext | low_priority | max | max_rows
|
match | mediumblob | mediumtext | mediumint
|
middleint | min_rows | minute | minute_second
|
modify | month | monthname | myisam
|
natural | numeric | no | not
|
null | on | optimize | option
|
optionally | or | order | outer
|
outfile | pack_keys | partial | password
|
precision | primary | procedure | process
|
processlist | privileges | read | real
|
references | reload | regexp | rename
|
replace | restrict | returns | revoke
|
rlike | row | rows | second
|
select | set | show | shutdown
|
smallint | soname | sql_big_tables | sql_big_selects
|
sql_low_priority_updates | sql_log_off | sql_log_update | sql_select_limit
|
sql_small_result | sql_big_result | sql_warnings | straight_join
|
starting | status | string | table
|
tables | temporary | terminated | text
|
then | time | timestamp | tinyblob
|
tinytext | tinyint | trailing | to
|
type | use | using | unique
|
unlock | unsigned | update | usage
|
values | varchar | variables | varying
|
varbinary | with | write | when
|
where | year | year_month | zerofill
|
The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them.
ACTION
BIT
DATE
ENUM
NO
TEXT
TIME
TIMESTAMP
This chapter provides a tutorial introduction to MySQL by showing
how to use the mysql
client program to create and use a simple
database. mysql
(sometimes referred to as the ``terminal monitor'' or
just ``monitor'') is an interactive program that allows you to connect to a
MySQL server, run queries and view the results. mysql
may
also be used in batch mode: you place your queries in a file beforehand, then
tell mysql
to execute the contents of the file. Both ways of using
mysql
are covered here.
To see a list of options provided by mysql
, invoke it with
the --help
option:
shell> mysql --help
This chapter assumes that mysql
is installed on your machine, and that
a MySQL server is available to which you can connect. If this is
not true, contact your MySQL administrator. (If you are the
administrator, you will need to consult other sections of this manual.)
The chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.
Since this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here.
To connect to the server, you'll usually need to provide a MySQL
user name when you invoke mysql
and, most likely, a password. If the
server runs on a machine other than the one where you log in, you'll also
need to specify a hostname. Contact your administrator to find out what
connection parameters you should use to connect (i.e., what host, user name
and password to use). Once you know the proper parameters, you should be
able to connect like this:
shell> mysql -h host -u user -p Enter password: ********
The ********
represents your password; enter it when mysql
displays the Enter password:
prompt.
If that works, you should see some introductory information followed by a
mysql>
prompt:
shell> mysql -h host -u user -p Enter password: ******** Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 459 to server version: 3.22.20a-log Type 'help' for help. mysql>
The prompt tells you that mysql
is ready for you to enter commands.
Some MySQL installations allow users to connect as the ``anonymous''
(unnamed) user to the server running on the local host. If this is the case
on your machine, you should be able to connect to that server by invoking
mysql
without any options:
shell> mysql
After you have connected successfully, you can disconnect any time by typing
QUIT
at the mysql>
prompt:
mysql> QUIT Bye
You can also disconnect by typing control-D.
Most examples in the following sections assume you are connected to the
server. They indicate this by the mysql>
prompt.
Make sure you are connected to the server, as discussed in the previous
section. Doing so will not in itself select any database to work with, but
that's okay. At this point, it's more important to find out a little about
how to issue queries than to jump right in creating tables, loading data
into them and retrieving data from them. This section describes the basic
principles of entering commands, using several queries you can try out to
familiarize yourself with how mysql
works.
Here's a simple command that asks the server to tell you its version number
and the current date. Type it in as shown below following the mysql>
prompt and hit the RETURN key:
mysql> SELECT VERSION(), CURRENT_DATE; +--------------+--------------+ | version() | CURRENT_DATE | +--------------+--------------+ | 3.22.20a-log | 1999-03-19 | +--------------+--------------+ 1 row in set (0.01 sec) mysql>
This query illustrates several things about mysql
:
QUIT
,
mentioned earlier, is one of them. We'll get to others later.)
mysql
sends it to the server for execution
and displays the results, then prints another mysql>
to indicate
that it is ready for another command.
mysql
displays query output as a table (rows and columns). The first
row contains labels for the columns. The rows following are the query
results. Normally, column labels are the names of the columns you fetch from
database tables. If you're retrieving the value of an expression rather than
a table column (as in the example just shown), mysql
labels the column
using the expression itself.
mysql
shows how many rows were returned, and how long the query took
to execute, which gives you a rough idea of server performance. These values
are imprecise because they represent wall clock time (not CPU or machine
time), and because they are affected by factors such as server load and
network latency. (For brevity, the ``rows in set'' line is not shown in
the remaining examples in this chapter.)
Keywords may be entered in any lettercase. The following queries are equivalent:
mysql> SELECT VERSION(), CURRENT_DATE; mysql> select version(), current_date; mysql> SeLeCt vErSiOn(), current_DATE;
Here's another query. It demonstrates that you can use mysql
as a
simple calculator:
mysql> SELECT SIN(PI()/4), (4+1)*5; +-------------+---------+ | SIN(PI()/4) | (4+1)*5 | +-------------+---------+ | 0.707107 | 25 | +-------------+---------+
The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon:
mysql> SELECT VERSION(); SELECT NOW(); +--------------+ | version() | +--------------+ | 3.22.20a-log | +--------------+ +---------------------+ | NOW() | +---------------------+ | 1999-03-19 00:15:33 | +---------------------+
A command need not be given all on a single line, so lengthy commands that
require several lines are not a problem. mysql
determines where your
statement ends by looking for the terminating semicolon, not by looking for
the end of the input line. (In other words, mysql
accepts free-format input: it collects input lines but does not execute them
until it sees the semicolon.)
Here's a simple multiple-line statement:
mysql> SELECT -> USER() -> , -> CURRENT_DATE; +--------------------+--------------+ | USER() | CURRENT_DATE | +--------------------+--------------+ | joesmith@localhost | 1999-03-18 | +--------------------+--------------+
In this example, notice how the prompt changes from mysql>
to
->
after you enter the first line of a multiple-line query. This is
how mysql
indicates that it hasn't seen a complete statement and is
waiting for the rest. The prompt is your friend, because it provides
valuable feedback. If you use that feedback, you will always be aware of
what mysql
is waiting for.
If you decide you don't want to execute a command that you are in the
process of entering, cancel it by typing \c
:
mysql> SELECT -> USER() -> \c mysql>
Here, too, notice the prompt. It switches back to mysql>
after you
type \c
, providing feedback to indicate that mysql
is ready
for a new command.
The following table shows each of the prompts you may see and summarizes what
they mean about the state that mysql
is in:
Prompt | Meaning |
mysql> | Ready for new command |
-> | Waiting for next line of multiple-line command |
'> | Waiting for next line, collecting a string that begins with a single quote (`'') |
"> | Waiting for next line, collecting a string that begins with a double quote (`"') |
Multiple-line statements commonly occur ``by accident'' when you intend to
issue a command on a single line, but forget the terminating semicolon. In
this case, mysql
waits for more input:
mysql> SELECT USER() ->
If this happens to you (you think you've entered a statement but the only
response is a ->
prompt), most likely mysql
is waiting for the
semicolon. If you don't notice what the prompt is telling you, you might sit
there for a while before realizing what you need to do. Enter a semicolon to
complete the statement, and mysql
will execute it:
mysql> SELECT USER() -> ; +--------------------+ | USER() | +--------------------+ | joesmith@localhost | +--------------------+
The '>
and ">
prompts occur during string collection.
In MySQL, you can write strings surrounded by either `''
or `"' characters (for example, 'hello'
or "goodbye"
),
and mysql
lets you enter strings that span multiple lines. When you
see a '>
or ">
prompt, it means that you've entered a line
containing a string that begins with a `'' or `"' quote character,
but have not yet entered the matching quote that terminates the string.
That's fine if you really are entering a multiple-line string, but how likely
is that? Not very. More often, the '>
and ">
prompts indicate
that you've inadvertantly left out a quote character. For example:
mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30; ">
If you enter this SELECT
statement, then hit RETURN and wait for the
result, nothing will happen. Instead of wondering, ``why does this
query take so long?,'' notice the clue provided by the ">
prompt. It
tells you that mysql
expects to see the rest of an unterminated
string. (Do you see the error in the statement? The string "Smith
is
missing the second quote.)
At this point, what do you do? The simplest thing is to cancel the command.
However, you cannot just type \c
in this case, because mysql
interprets it as part of the string that it is collecting! Instead, enter
the closing quote character (so mysql
knows you've finished the
string), then type \c
:
mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30; "> "\c mysql>
The prompt changes back to mysql>
, indicating that mysql
is ready for a new command.
It's important to know what the '>
and ">
prompts signify,
because if you mistakenly enter an unterminated string, any further lines you
type will appear to be ignored by mysql
-- including a line
containing QUIT
! This can be quite confusing, especially if you
don't know that you need to supply the terminating quote before you can
cancel the current command.
Here follows examples of how to solve some common problems with MySQL.
Some of the examples uses the table 'shop' holds the prices of each article (item number) for certain traders. Supposing that each trader has a single fixed price per article, then (item, trader) is a primary key for the records.
You can create the example table as:
CREATE TABLE shop ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, dealer CHAR(20) DEFAULT '' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL, PRIMARY KEY(article, dealer)); INSERT INTO shop VALUES (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69), (3,'D',1.25),(4,'D',19.95);
Okay, so the example data is:
SELECT * FROM shop +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | A | 3.45 | | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | B | 1.45 | | 0003 | C | 1.69 | | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+
"What's the highest item number?"
SELECT MAX(article) AS article FROM shop +---------+ | article | +---------+ | 4 | +---------+
"Find number, dealer, and price of the most expensive article"
In ANSI-SQL this is easily done with a sub-query:
SELECT article, dealer, price FROM shop WHERE price=(SELECT MAX(price) FROM shop)
In MySQL (not having sub-selects yet) just do it in two steps:
SELECT
statment.
SELECT article, dealer, price FROM shop WHERE price=19.95
Another solution is to sort all rows descending by price and only
get the first row using the MySQL specific LIMIT
clause:
SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1
Note: If there are several most expensive articles (e.g. each 19.95)
the LIMIT
solution does only show one of them!
"What's the highest price per article?"
SELECT article, MAX(price) AS price FROM shop GROUP BY article +---------+-------+ | article | price | +---------+-------+ | 0001 | 3.99 | | 0002 | 10.99 | | 0003 | 1.69 | | 0004 | 19.95 | +---------+-------+
"For each article find the dealer(s) with the most expensive price."
In ANSI SQL
I'd do it with a sub-query like this:
SELECT article, dealer, price FROM shop s1 WHERE price=(SELECT MAX(s2.price) FROM shop s2 WHERE s1.article = s2.article)
In MySQL it's best do it in several steps:
This can easily be done with a temporary table:
CREATE TEMPORARY TABLE tmp ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL); LOCK TABLES article read; INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article; SELECT article, dealer, price FROM shop, tmp WHERE shop.article=tmp.articel AND shop.price=tmp.price; UNLOCK TABLES; DROP TABLE tmp;
If you don't use a TEMPORARY
table, you must also lock the 'tmp' table.
"Can it be done with a single query?"
Yes, but only by using a quite inefficient trick that I call the "MAX-CONCAT trick":
SELECT article, SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer, 0.00+LEFT( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price FROM shop GROUP BY article; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | C | 1.69 | | 0004 | D | 19.95 | +---------+--------+-------+
The last example can of course be made a bit more efficient by doing the splitting of the concatenated column in the client.
One doesn't need foreign keys to join 2 tables.
The only thing MySQL
doesn't do is CHECK
to make sure that
the keys you use really exist in the table(s) you're referencing and it
doesn't automatically delete rows from table with a foreign key
definition. If you use your keys like normal, and it'll work just fine!
CREATE TABLE persons ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, name CHAR(60) NOT NULL, PRIMARY KEY (id) ); CREATE TABLE shirts ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, style ENUM('t-shirt', 'polo', 'dress') NOT NULL, color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL, owner SMALLINT UNSIGNED NOT NULL REFERENCES persons, PRIMARY KEY (id) ); INSERT INTO persons VALUES (NULL, 'Antonio Paz'); INSERT INTO shirts VALUES (NULL, 'polo', 'blue', LAST_INSERT_ID()), (NULL, 'dress', 'white', LAST_INSERT_ID()), (NULL, 't-shirt', 'blue', LAST_INSERT_ID()); INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska'); INSERT INTO shirts VALUES (NULL, 'dress', 'orange', LAST_INSERT_ID()), (NULL, 'polo', 'red', LAST_INSERT_ID()), (NULL, 'dress', 'blue', LAST_INSERT_ID()), (NULL, 't-shirt', 'white', LAST_INSERT_ID()); SELECT * FROM persons; +----+---------------------+ | id | name | +----+---------------------+ | 1 | Antonio Paz | | 2 | Lilliana Angelovska | +----+---------------------+ SELECT * FROM shirts; +----+---------+--------+-------+ | id | style | color | owner | +----+---------+--------+-------+ | 1 | polo | blue | 1 | | 2 | dress | white | 1 | | 3 | t-shirt | blue | 1 | | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | | 7 | t-shirt | white | 2 | +----+---------+--------+-------+ SELECT s.* FROM persons p, shirts s WHERE p.name LIKE 'Lilliana%' AND s.owner = p.id AND s.color <> 'white'; +----+-------+--------+-------+ | id | style | color | owner | +----+-------+--------+-------+ | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | +----+-------+--------+-------+
Now that you know how to enter commands, it's time to access a database.
Suppose you have several pets in your home (your ``menagerie'') and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows how to do all that:
The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records.
Use the SHOW
statement to find out what databases currently exist
on the server:
mysql> SHOW DATABASES; +----------+ | Database | +----------+ | mysql | | test | | tmp | +----------+
The list of databases is probably different on your machine, but the
mysql
and test
databases are likely to be among them. The
mysql
database is required because it describes user access
privileges. The test
database is often provided as a workspace for
users to try things out.
If the test
database exists, try to access it:
mysql> USE test Database changed
Note that USE
, like QUIT
, does not require a semicolon. (You
can terminate such statements with a semicolon if you like; it does no harm.)
The USE
statement is special in another way, too: it must be given on
a single line.
You can use the test
database (if you have access to it) for the
examples that follow, but anything you create in that database can be
removed by anyone else with access to it. For this reason, you should
probably ask your MySQL administrator for permission to use a
database of your own. Suppose you want to call yours menagerie
. The
administrator needs to execute a command like this:
mysql> GRANT ALL ON menagerie.* TO your_mysql_name;
where your_mysql_name
is the MySQL user name assigned to
you.
If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself:
mysql> CREATE DATABASE menagerie;
Under Unix, database names are case sensitive (unlike SQL keywords), so you
must always refer to your database as menagerie
, not as
Menagerie
, MENAGERIE
or some other variant. This is also true
for table names. (Under Windows, this restriction does not apply, although
you must refer to databases and tables using the same lettercase throughout a
given query.)
Creating a database does not select it for use, you must do that explicitly.
To make menagerie
the current database, use this command:
mysql> USE menagerie Database changed
Your database needs to be created only once, but you must select it for use
each time you begin a mysql
session. You can do this by issuing a
USE
statement as shown above. Alternatively, you can select the
database on the command line when you invoke mysql
. Just specify its
name after any connection parameters that you might need to provide. For
example:
shell> mysql -h host -u user -p menagerie Enter password: ********
Note that menagerie
is not your password on the command just shown.
If you want to supply your password on the command line after the -p
option, you must do so with no intervening space (e.g., as
-pmypassword
, not as -p mypassword
). However, putting your
password on the command line is not recommended, because doing so exposes it
to snooping by other users logged in on your machine.
Creating the database is the easy part, but at this point it's empty, as
SHOW TABLES
will tell you:
mysql> SHOW TABLES; Empty set (0.00 sec)
The harder part is deciding what the structure of your database should be: what tables you will need, and what columns will be in each of them.
You'll want a table that contains a record for each of your pets. This can
be called the pet
table, and it should contain, as a bare minimum,
each animal's name. Since the name by itself is not very interesting, the
table should contain other information. For example, if more than one person
in your family keeps pets, you might want to list each animal's owner. You
might also want to record some basic descriptive information such as species
and sex.
How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:
You can probably think of other types of information that would be useful in
the pet
table, but the ones identified so far are sufficient for now:
name, owner, species, sex, birth and death.
Use a CREATE TABLE
statement to specify the layout of your table:
mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20), -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);
VARCHAR
is a good choice for the name
, owner
and
species
columns since the column values will vary in length. The
lengths of those columns need not all be the same, and need not be
20
. You can pick any length from 1
to 255
, whatever
seems most reasonable to you. (If you make a poor choice and it turns
out later that you need a longer field, MySQL provides an
ALTER TABLE
statement.)
Animal sex can be represented in a variety of ways, for example, "m"
and "f"
, or perhaps "male"
and "female"
. It's simplest
to use the single characters "m"
and "f"
.
The use of the DATE
data type for the birth
and death
columns is a fairly obvious choice.
Now that you have created a table, SHOW TABLES
should produce some
output:
mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | pet | +---------------------+
To verify that your table was created the way you expected, use
a DESCRIBE
statement:
mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+
You can use DESCRIBE
any time, for example, if you forget the names of
the columns in your table or what types they are.
After creating your table, you need to populate it. The LOAD DATA
and
INSERT
statements are useful for this.
Suppose your pet records can be described as shown below.
(Observe that MySQL expects dates in YYYY-MM-DD
format;
this may be different than what you are used to.)
name | owner | species | sex | birth | death |
Fluffy | Harold | cat | f | 1993-02-04 | |
Claws | Gwen | cat | m | 1994-03-17 | |
Buffy | Harold | dog | f | 1989-05-13 | |
Fang | Benny | dog | m | 1990-08-27 | |
Bowser | Diane | dog | m | 1998-08-31 | 1995-07-29 |
Chirpy | Gwen | bird | f | 1998-09-11 | |
Whistler | Gwen | bird | 1997-12-09 | ||
Slim | Benny | snake | m | 1996-04-29 |
Since you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.
You could create a text file `pet.txt' containing one record per line,
with values separated by tabs, and given in the order in which the columns
were listed in the CREATE TABLE
statement. For missing values (such
as unknown sexes, or death dates for animals that are still living), you can
use NULL
values. To represent these in your text file, use
\N
. For example, the record for Whistler the bird would look like
this (where the whitespace between values is a single tab character):
Whistler | Gwen | bird | \N | 1997-12-09 | \N
|
To load the text file `pet.txt' into the pet
table, use this
command:
mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;
You can specify the column value separator and end of line marker explicitly
in the LOAD DATA
statement if you wish, but the defaults are tab and
linefeed. These are sufficient for the statement to read the file
`pet.txt' properly.
When you want to add new records one at a time, the INSERT
statement
is useful. In its simplest form, you supply values for each column, in the
order in which the columns were listed in the CREATE TABLE
statement.
Suppose Diane gets a new hamster named Puffball. You could add a new record
using an INSERT
statement like this:
mysql> INSERT INTO pet -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);
Note that string and date values are specified as quoted strings here. Also,
with INSERT
, you can insert NULL
directly to represent a
missing value. You do not use \N
like you do with LOAD DATA
.
From this example, you should be able to see that there would be a lot more
typing involved to load
your records initially using several INSERT
statements rather
than a single LOAD DATA
statement.
The SELECT
statement is used to pull information from a table.
The general form of the statement is:
SELECT what_to_select FROM which_table WHERE conditions_to_satisfy
what_to_select
indicates what you want to see. This can be a list of
columns, or *
to to indicate ``all columns.'' which_table
indicates the table from which you want to retrieve data. The WHERE
clause is optional. If it's present, conditions_to_satisfy
specifies
conditions that rows must satisfy to qualify for retrieval.
The simplest form of SELECT
retrieves everything from a table:
mysql> SELECT * FROM pet; +----------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+--------+---------+------+------------+------------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Fang | Benny | dog | m | 1990-08-27 | NULL | | Bowser | Diane | dog | m | 1998-08-31 | 1995-07-29 | | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+--------+---------+------+------------+------------+
This form of SELECT
is useful if you want to review your entire table,
for instance, after you've just loaded it with your initial dataset. As it
happens, the output just shown reveals an error in your data file: Bowser
appears to have been born after he died! Consulting your original pedigree
papers, you find that the correct birth year is 1989, not 1998.
There are are least a couple of ways to fix this:
DELETE
and LOAD DATA
:
mysql> DELETE FROM pet; mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;However, if you do this, you must also re-enter the record for Puffball.
UPDATE
statement:
mysql> UPDATE pet SET birth = "1989-08-31" WHERE name = "Bowser";
As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.
You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this:
mysql> SELECT * FROM pet WHERE name = "Bowser"; +--------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+-------+---------+------+------------+------------+ | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+-------+---------+------+------------+------------+
The output confirms that the year is correctly recorded now as 1989, not 1998.
String comparisons are normally case-insensitive, so you can specify the
name as "bowser"
, "BOWSER"
, etc. The query result will be
the same.
You can specify conditions on any column, not just name
. For example,
if you want to know which animals were born after 1998, test the birth
column:
mysql> SELECT * FROM pet WHERE birth >= "1998-1-1"; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+-------+---------+------+------------+-------+
You can combine conditions, for example, to locate female dogs:
mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
The preceding query uses the AND
logical operator. There is also an
OR
operator:
mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird"; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | +----------+-------+---------+------+------------+-------+
AND
and OR
may be intermixed. If you do that, it's a good idea
to use parentheses to indicate how conditions should be grouped:
mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m") -> OR (species = "dog" AND sex = "f"); +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
If you don't want to see entire rows from your table, just name the columns
in which you're interested, separated by commas. For example, if you want to
know when your animals were born, select the name
and birth
columns:
mysql> SELECT name, birth FROM pet; +----------+------------+ | name | birth | +----------+------------+ | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Buffy | 1989-05-13 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Puffball | 1999-03-30 | +----------+------------+
To find out who owns pets, use this query:
mysql> SELECT owner FROM pet; +--------+ | owner | +--------+ | Harold | | Gwen | | Harold | | Benny | | Diane | | Gwen | | Gwen | | Benny | | Diane | +--------+
However, notice that the query simply retrieves the owner
field from
each record, and some of them appear more than once. To minimize the output,
retrieve each unique output record just once by adding the keyword
DISTINCT
:
mysql> SELECT DISTINCT owner FROM pet; +--------+ | owner | +--------+ | Benny | | Diane | | Gwen | | Harold | +--------+
You can use a WHERE
clause to combine row selection with column
selection. For example, to get birth dates for dogs and cats only,
use this query:
mysql> SELECT name, species, birth FROM pet -> WHERE species = "dog" OR species = "cat"; +--------+---------+------------+ | name | species | birth | +--------+---------+------------+ | Fluffy | cat | 1993-02-04 | | Claws | cat | 1994-03-17 | | Buffy | dog | 1989-05-13 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | +--------+---------+------------+
You may have noticed in the preceding examples that the result rows are
displayed in no particular order. However, it's often easier to examine
query output when the rows are sorted in some meaningful way. To sort a
result, use an ORDER BY
clause.
Here are animal birthdays, sorted by date:
mysql> SELECT name, birth FROM pet ORDER BY birth; +----------+------------+ | name | birth | +----------+------------+ | Buffy | 1989-05-13 | | Bowser | 1989-08-31 | | Fang | 1990-08-27 | | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Slim | 1996-04-29 | | Whistler | 1997-12-09 | | Chirpy | 1998-09-11 | | Puffball | 1999-03-30 | +----------+------------+
To sort in reverse order, add the DESC
(descending) keyword to the
name of the column you are sorting by:
mysql> SELECT name, birth FROM pet ORDER BY birth DESC; +----------+------------+ | name | birth | +----------+------------+ | Puffball | 1999-03-30 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Claws | 1994-03-17 | | Fluffy | 1993-02-04 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Buffy | 1989-05-13 | +----------+------------+
You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query:
mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC; +----------+---------+------------+ | name | species | birth | +----------+---------+------------+ | Chirpy | bird | 1998-09-11 | | Whistler | bird | 1997-12-09 | | Claws | cat | 1994-03-17 | | Fluffy | cat | 1993-02-04 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | | Buffy | dog | 1989-05-13 | | Puffball | hamster | 1999-03-30 | | Slim | snake | 1996-04-29 | +----------+---------+------------+
Note that the DESC
keyword applies only to the column name immediately
preceding it (birth
); species
values are still sorted in
ascending order.
MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.
To determine how many years old each of your pets is, compute age as the difference between the birth date and the current date. Do this by converting the two dates to days, take the difference, and divide by 365 (the number of days in a year):
mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 FROM pet; +----------+-------------------------------------+ | name | (TO_DAYS(NOW())-TO_DAYS(birth))/365 | +----------+-------------------------------------+ | Fluffy | 6.15 | | Claws | 5.04 | | Buffy | 9.88 | | Fang | 8.59 | | Bowser | 9.58 | | Chirpy | 0.55 | | Whistler | 1.30 | | Slim | 2.92 | | Puffball | 0.00 | +----------+-------------------------------------+
Although the query works, there are some things about it that could be improved. First, the result could be scanned more easily if the rows were presented in some order. Second, the heading for the age column isn't very meaningful.
The first problem can be handled by adding an ORDER BY name
clause to
sort the output by name. To deal with the column heading, provide a name for
the column so that a different label appears in the output (this is called a
column alias):
mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 AS age -> FROM pet ORDER BY name; +----------+------+ | name | age | +----------+------+ | Bowser | 9.58 | | Buffy | 9.88 | | Chirpy | 0.55 | | Claws | 5.04 | | Fang | 8.59 | | Fluffy | 6.15 | | Puffball | 0.00 | | Slim | 2.92 | | Whistler | 1.30 | +----------+------+
To sort the output by age
rather than name
, just use a
different ORDER BY
clause:
mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 AS age -> FROM pet ORDER BY age; +----------+------+ | name | age | +----------+------+ | Puffball | 0.00 | | Chirpy | 0.55 | | Whistler | 1.30 | | Slim | 2.92 | | Claws | 5.04 | | Fluffy | 6.15 | | Fang | 8.59 | | Bowser | 9.58 | | Buffy | 9.88 | +----------+------+
A similar query can be used to determine age at death for animals that have
died. You determine which animals these are by checking whether or not the
death
value is NULL
. Then, for those with non-NULL
values, compute the difference between the death
and birth
values:
mysql> SELECT name, birth, death, (TO_DAYS(death)-TO_DAYS(birth))/365 AS age -> FROM pet WHERE death IS NOT NULL ORDER BY age; +--------+------------+------------+------+ | name | birth | death | age | +--------+------------+------------+------+ | Bowser | 1989-08-31 | 1995-07-29 | 5.91 | +--------+------------+------------+------+
The query uses death IS NOT NULL
rather than death != NULL
because NULL
is a special value. This is explained later.
See section 8.4.4.6 Working with NULL
values.
What if you want to know which animals have birthdays next month? For this
type of calculation, year and day are irrelevant, you simply want to extract
the month part of the birth
column. MySQL provides several
date-part extraction functions, such as YEAR()
, MONTH()
and
DAYOFMONTH()
. MONTH()
is the appropriate function here. To
see how it works, run a simple query that displays the value of both
birth
and MONTH(birth)
:
mysql> SELECT name, birth, MONTH(birth) FROM pet; +----------+------------+--------------+ | name | birth | MONTH(birth) | +----------+------------+--------------+ | Fluffy | 1993-02-04 | 2 | | Claws | 1994-03-17 | 3 | | Buffy | 1989-05-13 | 5 | | Fang | 1990-08-27 | 8 | | Bowser | 1989-08-31 | 8 | | Chirpy | 1998-09-11 | 9 | | Whistler | 1997-12-09 | 12 | | Slim | 1996-04-29 | 4 | | Puffball | 1999-03-30 | 3 | +----------+------------+--------------+
Finding animals with birthdays in the upcoming month is easy, too. Suppose
the current month is April. Then the month value is 4
and you look
for animals born in May (month 5) like this:
mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5; +-------+------------+ | name | birth | +-------+------------+ | Buffy | 1989-05-13 | +-------+------------+
There is a small complication if the current month is December, of course.
You don't just add one to the month number (12
) and look for animals
born in month 13, because there is no such month. Instead, you look for
animals born in January (month 1).
You can even write the query so that it works no matter what the current
month is. That way you don't have to use a particular month number
in the query. DATE_ADD()
allows you to add a time interval to a
given date. If you add a month to the value of NOW()
, then extract
the month part with MONTH()
, the result produces the month in which to
look for birthdays:
mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));
A different way to accomplish the same task is to add 1
to get the
next month after the current one (after using the modulo function (MOD
)
to ``wrap around'' the month value to 0
if it is currently
12
):
mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MOD(MONTH(NOW()),12) + 1;
NULL
values
The NULL
value can be surprising until you get used to it.
Conceptually, NULL
means ``missing value'' or ``unknown value'' and it
is treated somewhat differently than other values. To test for NULL
,
you cannot use the arithmetic comparison operators such as =
, <
or !=
. To demonstrate this for yourself, try the following query:
mysql> SELECT 1 = NULL, 1 != NULL, 1 < NULL, 1 > NULL; +----------+-----------+----------+----------+ | 1 = NULL | 1 != NULL | 1 < NULL | 1 > NULL | +----------+-----------+----------+----------+ | NULL | NULL | NULL | NULL | +----------+-----------+----------+----------+
Clearly you get no meaningful results from these comparisons. Use
the IS NULL
and IS NOT NULL
operators instead:
mysql> SELECT 1 IS NULL, 1 IS NOT NULL; +-----------+---------------+ | 1 IS NULL | 1 IS NOT NULL | +-----------+---------------+ | 0 | 1 | +-----------+---------------+
In MySQL, 0 means false and 1 means true.
This special treatment of NULL
is why, in the previous section, it
was necessary to determine which animals are no longer alive using
death IS NOT NULL
instead of death != NULL
.
MySQL provides standard SQL pattern matching as well as a form of
pattern matching based on extended regular expressions similar to those used
by Unix utilities such as vi
, grep
and sed
.
SQL pattern matching allows you to use `_' to match any single
character, and `%' to match an arbitrary number of characters (including
zero characters). In MySQL, SQL patterns are case insensitive by default.
Some examples are shown below. Note that you do not use =
or
!=
when you use SQL patterns; use the LIKE
or NOT LIKE
comparison operators instead.
To find names beginning with `b':
mysql> SELECT * FROM pet WHERE name LIKE "b%"; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+
To find names ending with `fy':
mysql> SELECT * FROM pet WHERE name LIKE "%fy"; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+
To find names containing a `w':
mysql> SELECT * FROM pet WHERE name LIKE "%w%"; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+
To find names containing exactly five characters, use the `_' pattern character:
mysql> SELECT * FROM pet WHERE name LIKE "_____"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
The other type of pattern matching provided by MySQL uses extended
regular expressions. When you test for a match for this type of pattern, use
the REGEXP
and NOT REGEXP
operators (or RLIKE
and
NOT RLIKE
, which are synonyms).
Some characteristics of extended regular expressions are:
To demonstrate how extended regular expressions work, the LIKE
queries
shown above are rewritten below to use REGEXP
:
To find names beginning with `b', use `^' to match the beginning of the name and `[bB]' to match either lowercase or uppercase `b':
mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]"; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+
To find names ending with `fy', use `$' to match the end of the name:
mysql> SELECT * FROM pet WHERE name REGEXP "fy$"; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+
To find names containing a `w', use `[wW]' to match either lowercase or uppercase `w':
mysql> SELECT * FROM pet WHERE name REGEXP "[wW]"; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+
Since a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wildcard on either side of the pattern to get it to match the entire value like it would be if you used an SQL pattern.
To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between:
mysql> SELECT * FROM pet WHERE name REGEXP "^.....$"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
You could also write the previous query using the `{n}'
``repeat-n
-times'' operator:
mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
Databases are often used to answer the question, ``how often does a certain type of data occur in a table?'' For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals.
Counting the total number of animals you have is the same question as ``how
many rows are in the pet
table?,'' since there is one record per pet.
The COUNT()
function counts the number of non-NULL
results, so
the query to count your animals looks like this:
mysql> SELECT COUNT(*) FROM pet; +----------+ | COUNT(*) | +----------+ | 9 | +----------+
Earlier, you retrieved the names of the people who owned pets. You can
use COUNT()
if you want to find out how many pets each owner has:
mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner; +--------+----------+ | owner | COUNT(*) | +--------+----------+ | Benny | 2 | | Diane | 2 | | Gwen | 3 | | Harold | 2 | +--------+----------+
Note the use of GROUP BY
to group together all records for each
owner
. Without it, all you get is an error message:
mysql> SELECT owner, COUNT(owner) FROM pet; ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...) with no GROUP columns is illegal if there is no GROUP BY clause
COUNT()
and GROUP BY
are useful for characterizing your
data in various ways. The following examples show different ways to
perform animal census operations.
Number of animals per species:
mysql> SELECT species, COUNT(*) FROM pet GROUP BY species; +---------+----------+ | species | COUNT(*) | +---------+----------+ | bird | 2 | | cat | 2 | | dog | 3 | | hamster | 1 | | snake | 1 | +---------+----------+
Number of animals per sex:
mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex; +------+----------+ | sex | COUNT(*) | +------+----------+ | NULL | 1 | | f | 4 | | m | 4 | +------+----------+
(In this output, NULL
indicates ``sex unknown.'')
Number of animals per combination of species and sex:
mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | NULL | 1 | | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+
You need not retrieve an entire table when you use COUNT()
. For
example, the previous query, when performed just on dogs and cats, looks like
this:
mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE species = "dog" OR species = "cat" -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | +---------+------+----------+
Or, if you wanted the number of animals per sex only for known-sex animals:
mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE sex IS NOT NULL -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+
The pet
table keeps track of which pets you have. If you want to
record other information about them, such as events in their lives like
visits to the vet or when litters are born, you need another table. What
should this table look like?
Given these considerations, the CREATE TABLE
statement for the
event
table might look like this:
mysql> CREATE TABLE event (name VARCHAR(20), date DATE, -> type VARCHAR(15), remark VARCHAR(255));
As with the pet
table, it's easiest to load the initial records
by creating a tab-delimited text file containing the information:
Fluffy | 1995-05-15 | litter | 4 kittens, 3 female, 1 male |
Buffy | 1993-06-23 | litter | 5 puppies, 2 female, 3 male |
Buffy | 1994-06-19 | litter | 3 puppies, 3 female |
Chirpy | 1999-03-21 | vet | needed beak straightened |
Slim | 1997-08-03 | vet | broken rib |
Bowser | 1991-10-12 | kennel | |
Fang | 1991-10-12 | kennel | |
Fang | 1998-08-28 | birthday | Gave him a new chew toy |
Claws | 1998-03-17 | birthday | Gave him a new flea collar |
Whistler | 1998-12-09 | birthday | First birthday |
Load the records like this:
mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;
Based on what you've learned from the queries you've run on the pet
table, you should be able to perform retrievals on the records in the
event
table; the principles are the same. But when is the
event
table by itself insufficient to answer questions you might ask?
Suppose you want to find out the ages of each pet when they had their
litters. The event
table indicates when this occurred, but to
calculate age of the mother, you need her birth date. Since that is
stored in the pet
table, you need both tables for the query:
mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark -> FROM pet, event -> WHERE pet.name = event.name AND type = "litter"; +--------+------+-----------------------------+ | name | age | remark | +--------+------+-----------------------------+ | Fluffy | 2.27 | 4 kittens, 3 female, 1 male | | Buffy | 4.12 | 5 puppies, 2 female, 3 male | | Buffy | 5.10 | 3 puppies, 3 female | +--------+------+-----------------------------+
There are several things to note about this query:
FROM
clause lists two tables since the query needs to pull
information from both of them.
name
column. The query uses
WHERE
clause to match up records in the two tables based on the
name
values.
name
column occurs in both tables, you must be specific
about which table you mean when referring to the column. This is done
by prepending the table name to the column name.
You need not have two different tables to perform a join. Sometimes it is
useful to join a table to itself, if you want to compare records in a table
to other records in that same table. For example, to find breeding pairs
among your pets, you can join the pet
table with itself to pair up
males and females of like species:
mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species -> FROM pet AS p1, pet AS p2 -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m"; +--------+------+--------+------+---------+ | name | sex | name | sex | species | +--------+------+--------+------+---------+ | Fluffy | f | Claws | m | cat | | Buffy | f | Fang | m | dog | | Buffy | f | Bowser | m | dog | +--------+------+--------+------+---------+
In this query, we specify aliases for the table name in order to be able to refer to the columns and keep straight which instance of the table each column reference is associated with.
What if you forget the name of a database or table, or what the structure of a given table is (e.g., what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.
You have already seen SHOW DATABASES
, which lists the databases
managed by the server. To find out which database is currently selected,
use the DATABASE()
function:
mysql> SELECT DATABASE(); +------------+ | DATABASE() | +------------+ | menagerie | +------------+
If you haven't selected any database yet, the result is blank.
To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command:
mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | event | | pet | +---------------------+
If you want to find out about the structure of a table, the DESCRIBE
command is useful; it displays information about each of a table's columns:
mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+
Field
indicates the column name, Type
is the data type for
the column, Null
indicates whether or not the column can contain
NULL
values, Key
indicates whether or not the column is
indexed and Default
specifies the column's default value.
If you have indexes on a table,
SHOW INDEX FROM tbl_name
produces information about them.
mysql
in batch mode
In the previous sections, you used mysql
interactively to enter
queries and view the results. You can also run mysql
in batch
mode. To do this, put the commands you want to run in a file, then
tell mysql
to read its input from the file:
shell> mysql < batch-file
If you need to specify connection parameters on the command line, the command might look like this:
shell> mysql -h host -u user -p < batch-file Enter password: ********
When you use mysql
this way, you are creating a script file, then
executing the script.
Why use a script? Here are a few reasons:
mysql
to execute it again.
shell> mysql < batch-file | more
shell> mysql < batch-file > mysql.out
cron
job. In this case, you must use batch mode.
The default output format is different (more concise) when you run
mysql
in batch mode than when you use it interactively. For
example, the output of SELECT DISTINCT species FROM pet
looks like
this when run interactively:
+---------+ | species | +---------+ | bird | | cat | | dog | | hamster | | snake | +---------+
But like this when run in batch mode:
species bird cat dog hamster snake
If you want to get the interactive output format in batch mode, use
mysql -t
. To echo to the output the commands that are executed, use
mysql -vvv
.
At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.
The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.
More information about Twin studies can be found at:
http://www.imm.ki.se/TWIN/TWINUKW.HTM
The latter part of the project is administered with a web interface written using Perl and MySQL.
Each night all data from the interviews are moved into a MySQL database.
The following query is used to determine who goes into the second part of the project:
select concat(p1.id, p1.tvab) + 0 as tvid, concat(p1.christian_name, " ", p1.surname) as Name, p1.postal_code as Code, p1.city as City, pg.abrev as Area, if(td.participation = "Aborted", "A", " ") as A, p1.dead as dead1, l.event as event1, td.suspect as tsuspect1, id.suspect as isuspect1, td.severe as tsevere1, id.severe as isevere1, p2.dead as dead2, l2.event as event2, h2.nurse as nurse2, h2.doctor as doctor2, td2.suspect as tsuspect2, id2.suspect as isuspect2, td2.severe as tsevere2, id2.severe as isevere2, l.finish_date from twin_project as tp /* For Twin 1 */ left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab left join harmony as h on tp.id = h.id and tp.tvab = h.tvab left join lentus as l on tp.id = l.id and tp.tvab = l.tvab /* For Twin 2 */ left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab, person_data as p1, person_data as p2, postal_groups as pg where /* p1 gets main twin and p2 gets his/her twin. */ /* ptvab is a field inverted from tvab */ p1.id = tp.id and p1.tvab = tp.tvab and p2.id = p1.id and p2.ptvab = p1.tvab and /* Just the sceening survey */ tp.survey_no = 5 and /* Skip if partner died before 65 but allow emigration (dead=9) */ (p2.dead = 0 or p2.dead = 9 or (p2.dead = 1 and (p2.death_date = 0 or (((to_days(p2.death_date) - to_days(p2.birthday)) / 365) >= 65)))) and ( /* Twin is suspect */ (td.future_contact = 'Yes' and td.suspect = 2) or /* Twin is suspect - Informant is Blessed */ (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or /* No twin - Informant is Blessed */ (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or /* Twin broken off - Informant is Blessed */ (td.participation = 'Aborted' and id.suspect = 1 and id.future_contact = 'Yes') or /* Twin broken off - No inform - Have partner */ (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0)) and l.event = 'Finished' /* Get at area code */ and substring(p1.postal_code, 1, 2) = pg.code /* Not already distributed */ and (h.nurse is NULL or h.nurse=00 or h.doctor=00) /* Has not refused or been aborted */ and not (h.status = 'Refused' or h.status = 'Aborted' or h.status = 'Died' or h.status = 'Other') order by tvid;
Some explanations:
concat(p1.id, p1.tvab) + 0 as tvid
id
and tvab
in
numerical order. Adding 0
to the result causes MySQL to
treat the result as a number.
id
tvab
1
or 2
.
ptvab
tvab
. When tvab
is 1
this is
2
, and vice versa. It exists to save typing and to make it easier for
MySQL to optimize the query.
This query demonstrates, among other things, how to do lookups on a
table from the same table with a join (p1
and p2
). In the example, this
is used to check whether a twin's partner died before the age of 65. If so,
the row is not returned.
All of the above exist in all tables with twin-related information. We
have a key on both id,tvab
(all tables) and id,ptvab
(person_data
) to make queries faster.
On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.
The current number of records in the tables used above:
Table | Rows |
person_data | 71074 |
lentus | 5291 |
twin_project | 5286 |
twin_data | 2012 |
informant_data | 663 |
harmony | 381 |
postal_groups | 100 |
Each interview ends with a status code called event
. The query
shown below is used to display a table over all twin pairs combined by
event. This indicates in how many pairs both twins are finished, in how many
pairs one twin is finished and the other refused, and so on.
select t1.event, t2.event, count(*) from lentus as t1, lentus as t2, twin_project as tp where /* We are looking at one pair at a time */ t1.id = tp.id and t1.tvab=tp.tvab and t1.id = t2.id /* Just the sceening survey */ and tp.survey_no = 5 /* This makes each pair only appear once */ and t1.tvab='1' and t2.tvab='2' group by t1.event, t2.event;
mysqld
can issue error messages in the following languages: Czech,
Dutch, English (the default), Estonia, French, German, Hungarian, Italian,
Norwegian, Norwegian-ny, Polish, Portuguese, Spanish and Swedish.
To start mysqld
with a particular language, use either the
--language=lang
or -L lang
options. For example:
shell> mysqld --language=swedish
or:
shell> mysqld --language=/usr/local/share/swedish
Note that all language names are specified in lowercase.
The language files are located (by default) in `mysql_base_dir/share/LANGUAGE/'.
To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file:
shell> comp_err errmsg.txt errmsg.sys
If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.
By default, MySQL uses the ISO-8859-1 (Latin1) character set. This is the character set used in the USA and western Europe.
The character set determines what characters are allowed in names and how
things are sorted by the ORDER BY
and GROUP BY
clauses of
the SELECT
statement.
You can change the character set at compile time by using the
--with-charset=charset
option to configure
.
See section 4.7.1 Quick installation overview.
To add another character set to MySQL, use the following procedure:
MYSET
below.
ctype_MYSET
, to_lower_MYSET
and so on.
to_lower[]
and to_upper[]
are simple arrays that hold the
lowercase and uppercase characters corresponding to each member of the
character set. For example:
to_lower['A'] should contain 'a' to_upper['a'] should contain 'A'
sort_order[]
is a map indicating how characters should be ordered for
comparison and sorting purposes. For many character sets, this is the same as
to_upper[]
(which means sorting will be case insensitive).
MySQL will sort characters based on the value of
sort_order[character]
.
ctype[]
is an array of bit values, with one element for one character.
(Note that to_lower[]
, to_upper[]
and sort_order[]
are indexed by character value, but ctype[]
is indexed by character
value + 1. This is an old legacy to be able to handle EOF.)
You can find the following bitmask definitions in `m_ctype.h':
#define _U 01 /* Upper case */ #define _L 02 /* Lower case */ #define _N 04 /* Numeral (digit) */ #define _S 010 /* Spacing character */ #define _P 020 /* Punctuation */ #define _C 040 /* Control character */ #define _B 0100 /* Blank */ #define _X 0200 /* heXadecimal digit */The
ctype[]
entry for each character should be the union of the
applicable bitmask values that describe the character.
For example, 'A'
is an uppercase character (_U
) as well as a
hexadecimal digit (_X
), so ctype['A'+1]
should contain the value:
_U + _X = 01 + 0200 = 0201
CHARSETS_AVAILABLE
list in
configure.in
.
If you are creating a multi-byte character set, you can use the
_MB
macros. In `include/m_ctype.h.in', add:
#define MY_CHARSET_MYSET X #if MY_CHARSET_CURRENT == MY_CHARSET_MYSET #define USE_MB #define USE_MB_IDENT #define ismbchar(p, end) (...) #define ismbhead(c) (...) #define mbcharlen(c) (...) #define MBMAXLEN N #endif
Where:
MY_CHARSET_MYSET | A unique character set value. |
USE_MB | This character set has multi-byte
characters, handled by ismbhead() and mbcharlen()
|
USE_MB_IDENT | (optional) If defined, you can use table and column names that use multi-byte characters |
ismbchar(p, e) | return 0 if p is not a multi-byte
character string, or the size of the character (in bytes) if it is. p
and e point to the beginning and end of the string.
Check from (char*)p to (char*)e-1 .
|
ismbhead(c) | True if c is the first character of
a multi-byte character string
|
mbcharlen(c) | Size of a multi-byte character string if
c is the first character of such a string
|
MBMAXLEN | Size in bytes of the largest character in the set |
When started with the --log-update=file_name
option, mysqld
writes a log file containing all SQL commands that update data. The file is
written in the data directory and has a name of file_name.#
, where
#
is a number that is incremented each time you execute
mysqladmin refresh
or mysqladmin flush-logs
, the FLUSH
LOGS
statement, or restart the
server.
If you use the --log
or -l
options, mysqld
writes a
general log with a filename of `hostname.log', and restarts and
refreshes do not cause a new log file to be generated (although it is closed
and reopened). By default, the mysql.server
script starts the
MySQL server with the -l
option. If you need better
performance when you start using MySQL in a production environment,
you can remove the -l
option from mysql.server
.
Update logging is smart since it logs only statements that really update
data. So an UPDATE
or a DELETE
with a WHERE
that finds no
rows is not written to the log. It even skips UPDATE
statements that
set a column to the value it already has.
If you want to update a database from update log files, you could do the following (assuming your update logs have names of the form `file_name.#'):
shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql
ls
is used to get all the log files in the right order.
This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.
You can also use the update logs when you have a mirrored database on another host and you want to replicate the changes that have been made to the master database.
MySQL 3.22 has a 4G limit on table size. With the new MyISAM in MySQL 3.23 the maximum table size is pushed up to 8 million terabytes (2 ^ 63 bytes).
Note however that operating systems have their own file size limits. On Linux, the current limit is 2G; on Solaris 2.5.1, the limit is 4G; on Solaris 2.6, the limit is 1000G. This means that the table size for MySQL is normally limited by the operating system.
By default, MySQL tables have a maximum size of about 4G. You can
check the maximum table size for a table with the SHOW TABLE STATUS
command or with the myisamchk -dv table_name
.
See section 7.21 SHOW
syntax (Get information about tables, columns,...).
If you need bigger tables than 4G (and your operating system supports
this), you should set the AVG_ROW_LENGTH
and MAX_ROWS
parameter when you create your table. See section 7.7 CREATE TABLE
syntax. You can
also set these later with ALTER TABLE
. See section 7.8 ALTER TABLE
syntax.
If your big table is going to be read-only, you could use
myisampack
to merge and compress many tables to one.
myisampack
usually compresses a table by at least 50%, so you can
have, in effect, much bigger tables. See section 12.5 The MySQL compressed read-only table generator.
Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. (Identical in this case means that all tables are created with identical column information.) Currently MERGE can only be used to scan a collection of tables because it doesn't support indexes. We will add indexes to this in the near future.
With MySQL you can currently (version 3.23.5) choose between tree basic
table formats. When you create a new table, you can tell MySQL
which table type it should use for the table. MySQL will
always create a .frm
file to hold the table and column
definitions. Depending on the table type the index and data will be
stored in other files.
You can convert tables between different types with the ALTER
TABLE
statement. See section 7.8 ALTER TABLE
syntax.
MyISAM
}
MyISAM
is the default table type in MySQL 3.23. It's
based on the ISAM
code and has a lot of useful extensions.
The index is stored in a file with the .MYI
(MYindex) extension
and the data is stored in file with the .MYD
(MYData) extension.
You can check/repair MyISAM
tables with the myisamchk
utility. See section 13.4 Using myisamchk
for crash recovery.
The following is new in MyISAM
:
AUTO_INCREMENT
column. MyISAM
will automatically update this on INSERT/UPDATE
. The
AUTO_INCREMENT
value can be reset with myisamchk
. This
will make AUTO_INCREMENT
columns faster and old numbers will not
be reused as with the old ISAM. Not that when a AUTO_INCREMENT
is defined on the end of a key the old behavior is still present.
BLOB
and TEXT
columns can be indexed.
NULL
values are allowed in indexed columns. This takes 0-1
bytes/key.
myisamchk
.
MyISAM
file that indicates whether or not the
table was closed correctly. This will soon be used for automatic repair
in the MySQL server.
myisamchk
will now mark tables as checked. myisamchk
--fast
will only check those tables that don't have this mark.
myisamchk -a
stores statistics for key parts (and not only for
whole keys as in ISAM
).
myisampack
can pack BLOB
and VARCHAR
columns.
MyISAM
also supports the following things, which MySQL
will be able to use in the near future.
VARCHAR
type; A VARCHAR
column starts
with a length stored in 2 bytes.
VARCHAR
may have fixed or dynamic record length.
VARCHAR
and CHAR
may be up to 64K.
All key segments have their own language definition. This will enable
MySQL to have different language definitions per column.
UNIQUE
; This will allow
you to have UNIQUE
on any combination of columns in a table. (You
can't search on a UNIQUE
computed index, however.)
MyISAM
is a better implementation of the same
thing. ISAM uses a B-tree
index. The index is stored in a file
with the .ISM
extension and the data is stored in file with the
.ISD
extension. You can check/repair ISAM tables with the
isamchk
utility. See section 13.4 Using myisamchk
for crash recovery. ISAM tables are not
binary portable across OS/Platforms.
ISAM
has the following features/properties:
HEAP
}
HEAP
tables use a hashed index and are stored in memory. This
makes them very fast, but if MySQL crashes you will lose all
data stored in them. HEAP
is very usable as temporary tables!
CREATE TABLE test TYPE=HEAP SELECT ip,SUM(downloads) as down FROM log_table GROUP BY ip; SELECT count(ip),avg(down) from test; DROP TABLE test;Here are some things you should consider when you use
HEAP
tables:
MAX_ROWS
in the CREATE
statement
to ensure that you accidently do not use all memory.
=
and <=>
(but are VERY fast).
HEAP
tables uses a fixed record length format.
HEAP
doesn't support BLOB
/TEXT
columns.
HEAP
doesn't support AUTO_INCREMENT
columns.
HEAP
doesn't support an index on a NULL
column.
HEAP
table (not that normal
with hashed tables).
HEAP
tables are shared between all clients (just like any other
table).
HEAP
tables are allocated in small blocks. The tables
are 100% dynamic (on inserting). No overflow areas and no extra key
space is needed. Deleted rows are put in a linked list and will be
reused when you insert new data into the table.
DELETE FROM heap_table
or
DROP TABLE heap_table
.
HEAP
tables bigger than max_heap_table_size
.
Optimization is a complicated task since it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system/application, the more optimal you want your system to become the more you will have to know about it.
So this chapter will try to explain and give some examples of different ways to optimize MySQL. But remember that there are always some (increasingly harder) ways to make the system even faster left to do.
The most important part for getting a system fast is of course the basic design. You also need to know that kinds of things your system will be doing. That is your bottlenecks are.
The most common bottlenecks are.
We start with the system level things sine some of these decisions have to be made very early. In other cases a fast look at this part may suffice since it not that important for the big gains. But it is always nice to have a feeling about how much one gould gain by chancing things at this level.
The default OS to use is really important! To get the most use of multiple CPU machines one should use Solaris (because the threads works really nice) or Linux (because the 2.2 kernel has really good SMP support). Also on 32bit machines Linux has a 2G file size limit by default. Hopefully this will be fixed soon when new filesystems is released (XFS).
Since we have not run production MySQL on that many platforms we advice you to test your intended platform before choosing it if possible.
Other tips:
--skip-locking
MySQL option to avoid external
locking. Note that this will not impact MySQL functionality as long that
only run one server. Just remember to take down the server (or lock
relevant parts) before you run myisamchk
. On some system this
switch is mandatory since the external locking does not work in any
case.
The --skip-locking
option is on by default when compiling with
MIT-pthreads, because flock()
isn't fully supported by
MIT-pthreads on all platforms.
The only case when you can't use --skip-locking
is if you run
multiple MySQL SERVERS (not clients) on the same data. Or run
myisamchk
on the table without first flushing and locking the
mysqld
server tables first.
You can still use LOCK TABLES
/ UNLOCK TABLES
even if you
are using --skip-locking
Most of the following tests are done on Linux and with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.
You get the fastest executable when you link with -static
. Using Unix
sockets rather than TCP/IP to connect to a database also gives better
performance.
On Linux, you will get the fastest code when compiling with pgcc
and -O6
. To compile `sql_yacc.cc' with these options, you
need about 200M memory because gcc/pgcc
needs a lot of memory to
make all functions inline. You should also set CXX=gcc
when
configuring MySQL to avoid inclusion of the libstdc++
library (it is not needed).
By just using a better compiler and/or better compiler options you can get a 10-30 % speed increase in your application. This is particularly important if you compile the SQL server yourselves!
On Intel you should for example use pgcc or the Cygnus gnu pro compiler to get maximum speed. We have tested the new Fujitsu compiler but it is not yet bug free enough to compile MySQL with optimizations on.
Here is a list of some mesurements that we have done:
pgcc
and compile everything with -O6
, the
mysqld
server is 11% faster than with gcc
(with versions
as of string 99).
-static
), the result is 13%
slower. Note that you still can use a dynamic linked MySQL library. It
is only the server that is critical for performance.
gcc
2.7.3 is 13% faster than Sun Pro C++ 4.2.
The MySQL-Linux distribution provided by TcX is compiled with
pgcc
and linked statically.
You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space.
If MySQL notices that a table is a symbolically-linked, it will
resolve the symlink and use the table it points to instead. This works
on all systems that support the realpath()
call (at least Linux
and Solaris support realpath()
)! On systems that don't support
realpath()
, you should not access the table through the real path
and through the symlink at the same time! If you do, the table will be
inconsistent after any update.
MySQL doesn't support linking of databases by default. Things
will work fine as long as you don't make a symbolic link between
databases. Suppose you have a database db1
under the
MySQL data directory, and then make a symlink db2
that
points to db1
:
shell> cd /path/to/datadir shell> ln -s db1 db2
Now, for any table tbl_a
in db1
, there also appears to be
a table tbl_a
in db2
. If one thread updates db1.tbl_a
and another thread updates db2.tbl_a
, there will be problems.
If you really need this, you must change the following code in `mysys/mf_format.c':
if (!lstat(to,&stat_buff)) /* Check if it's a symbolic link */ if (S_ISLNK(stat_buff.st_mode) && realpath(to,buff))
Change the code to this:
if (realpath(to,buff))
You can get the default buffer sizes used by the mysqld
server
with this command:
shell> mysqld --help
This command produces a list of all mysqld
options and configurable
variables. The output includes the default values and looks something
like this:
Possible variables for option --set-variable (-O) are: back_log current value: 5 connect_timeout current value: 5 delayed_insert_timeout current value: 300 delayed_insert_limit current value: 100 delayed_queue_size current value: 1000 flush_time current value: 0 join_buffer_size current value: 131072 key_buffer_size current value: 1048540 long_query_time current value: 10 max_allowed_packet current value: 1048576 max_connections current value: 100 max_connect_errors current value: 10 max_delayed_threads current value: 20 max_heap_table_size current value: 16777216 max_join_size current value: 4294967295 max_sort_length current value: 1024 max_tmp_tables current value: 32 net_buffer_length current value: 16384 record_buffer current value: 131072 sort_buffer current value: 2097116 table_cache current value: 64 tmp_table_size current value: 1048576 thread_stack current value: 131072 wait_timeout current value: 28800
If there is a mysqld
server currently running, you can see what
values it actually is using for the variables by executing this command:
shell> mysqladmin variables
Each option is described below. Values for buffer sizes, lengths and stack
sizes are given in bytes. You can specify values with a suffix of `K'
or `M' to indicate kilobytes or megabytes. For example, 16M
indicates 16 megabytes. Case of suffix letters does not matter;
16M
and 16m
are equivalent.
You can also see some statistics from a running server by the command
SHOW STATUS
. See section 7.21 SHOW
syntax (Get information about tables, columns,...).
back_log
back_log
value indicates how many requests can be
stacked during this short time before MySQL momentarily stops
answering new requests. You need to increase this only if you expect a large
number of connections in a short period of time.
In other words, this value is the size of the listen queue for incoming
TCP/IP connections. Your operating system has its own limit on the size
of this queue. The manual page for the Unix listen(2)
system
call should have more details. Check your OS documentation for the
maximum value for this variable. Attempting to set back_log
higher than your operating system limit will be ineffective.
connect_timeout
mysqld
server is waiting for a connect
packet before responding with Bad handshake
.
delayed_insert_timeout
INSERT DELAYED
thread should wait for INSERT
statements before terminating.
delayed_insert_limit
delayed_insert_limit
rows, the INSERT
DELAYED
handler will check if there are any SELECT
statements
pending. If so, it allows these to execute before continuing.
delayed_queue_size
INSERT DELAYED
.
If the queue becomes full, any client that does INSERT DELAYED
will wait until
there is room in the queue again.
flush_time
flush_time
seconds all
tables will be closed (to free up resources and sync things to disk).
join_buffer_size
key_buffer_size
key_buffer_size
is the size of the buffer used for index blocks.
Increase this get better index handling (for all reads and multiple
writes) to as much as you can afford. If you make this too big the
system will starte to page and go REAL slow. Remember that since MySQL
does not cache data read that you will have to leave some room for the
OS filesystem cache.
To get even more speed for writes use LOCK TABLES
. See section 7.24 LOCK TABLES/UNLOCK TABLES
syntax.
long_query_time
Slow_queries
counter
will be incremented.
max_allowed_packet
net_buffer_length
bytes, but can grow up to max_allowed_packet
bytes when needed. This value by default is small to catch big (possibly
wrong) packets. You must increase this value if you are using big
BLOB
columns. It should be as big as the biggest BLOB
you want
to use.
max_connections
mysqld
requires. See below for
comments on file descriptor limits.
max_connect_errors
FLUSH HOSTS
.
max_delayed_threads
INSERT DELAYED
statements. If you try to insert data in a new table after all INSERT
DELAYED
threads are in use, the row will be inserted as if the
DELAYED
attribute wasn't specified.
max_join_size
max_join_size
records return an error. Set this value if your users tend to perform joins
without a WHERE
clause that take a long time and return
millions of rows.
max_sort_length
BLOB
or TEXT
values (only the first max_sort_length
bytes of each value
are used; the rest are ignored).
max_tmp_tables
net_buffer_length
max_allowed_packet
bytes.)
record_buffer
sort_buffer
ORDER BY
or GROUP BY
operations.
See section 18.5 Where MySQL stores temporary files.
table_cache
mysqld
requires. MySQL
needs two file descriptors for each unique open table. See below for
comments on file descriptor limits. For information about how the table
cache works, see section 10.2.4 How MySQL opens and closes tables.
tmp_table_size
The table tbl_name is full
. Increase the value of
tmp_table_size
if you do many advanced GROUP BY
queries.
thread_stack
crash-me
test are dependent on this value. The default is
normally large enough for normal operation. See section 10.8 Using your own benchmarks.
wait_timeout
MySQL uses algorithms that are very scalable, so you can usually run with very little memory or give MySQL more memory to get better performance.
If you have much memory and many tables and want maximum performance with a moderate number of clients, you should use something like this:
shell> safe_mysqld -O key_buffer=16M -O table_cache=128 \ -O sort_buffer=4M -O record_buffer=1M &
If you have little memory and lots of connections, use something like this:
shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \ -O record_buffer=100k &
or even:
shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \ -O table_cache=32 -O record_buffer=8k -O net_buffer=1K &
If there are very many connections, ``swapping problems'' may occur unless
mysqld
has been configured to use very little memory for each
connection. mysqld
performs better if you have enough memory for all
connections, of course.
Note that if you change an option to mysqld
, it remains in effect only
for that instance of the server.
To see the effects of a parameter change, do something like this:
shell> mysqld -O key_buffer=32m --help
Make sure that the --help
option is last; otherwise, the effect of any
options listed after it on the command line will not be reflected in the
output.
table_cache
, max_connections
and max_tmp_tables
affect the maximum number of files the server keeps open. If you
increase one or both of these values, you may run up against a limit
imposed by your operating system on the per-process number of open file
descriptors. However, you can increase the limit on many systems.
Consult your OS documentation to find out how to do this, because the
method for changing the limit varies widely from system to system.
table_cache
is related to max_connections
.
For example, for 200 open connections, you should have a table cache of
at least 200 * n
, where n
is the maximum number of tables in
a join.
The cache of open tables can grow to a maximum of table_cache
(default 64; this can be changed with with the -O table_cache=#
option to mysqld
). A table is never closed, except when the
cache is full and another thread tries to open a table or if you use
mysqladmin refresh
or mysqladmin flush-tables
.
When the table cache fills up, the server uses the following procedure to locate a cache entry to use:
A table is opened for each concurrent access. This means that
if you have two threads accessing the same table or access the table
twice in the same query (with AS
) the table needs to be opened twice.
The first open of any table takes two file descriptors; each additional
use of the table takes only one file descriptor. The extra descriptor
for the first open is used for the index file; this descriptor is shared
among all threads.
If you have many files in a directory, open, close and create operations will
be slow. If you execute SELECT
statements on many different tables,
there will be a little overhead when the table cache is full, because for
every table that has to be opened, another must be closed. You can reduce
this overhead by making the table cache larger.
When you run mysqladmin status
, you'll see something like this:
Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12
This can be somewhat perplexing if you only have 6 tables.
MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.
The list below indicates some of the ways that the mysqld
server
uses memory. Where applicable, the name of the server variable relevant
to the memory use is given.
key_buffer_size
) is shared by all
threads; Other buffers used by the server are allocated as
needed. See section 10.2.3 Tuning server parameters.
thread_stack
) a connection buffer (variable
net_buffer_length
), and a result buffer (variable
net_buffer_length
). The connection buffer and result buffer are
dynamically enlarged up to max_allowed_packet
when needed. When
a query is running a copy of the current query string is also allocated.
record_buffer
).
BLOB
columns are
stored on disk.
One problem in MySQL versions before 3.32.2 is that if a HEAP table
exceeds the size of tmp_table_size
, you get the error The
table tbl_name is full
. In newer versions this is handled by
automatically changing the in-memory (HEAP) table to a disk-based
(MyISAM) table as necessary. To work around this problem, you can
increase the temporary table size by setting the tmp_table_size
option to mysqld
, or by setting the SQL option
SQL_BIG_TABLES
in the client program. See section 7.25 SET OPTION
syntax. In MySQL 3.20, the maximum size of the
temporary table was record_buffer*16
, so if you are using this
version, you have to increase the value of record_buffer
. You can
also start mysqld
with the --big-tables
option to always
store temporary tables on disk, however, this will affect the speed of
all complicated queries.
malloc()
and
free()
).
3 * n
is
allocated (where n
is the maximum row length, not counting BLOB
columns). A BLOB
uses 5 to 8 bytes plus the length of the BLOB
data.
BLOB
columns, a buffer is enlarged dynamically
to read in larger BLOB
values. If you scan a table, a buffer as large
as the largest BLOB
value is allocated.
mysqladmin flush-tables
command closes all tables that are not in
use and marks all in-use tables to be closed when the currently executing
thread finishes. This will effectively free most in-use memory.
ps
and other system status programs may report that mysqld
uses a lot of memory. This may be caused by thread-stacks on different
memory addresses. For example, the Solaris version of ps
counts
the unused memory
between stacks as used memory. You can verify this by checking available
swap with swap -s
. We have tested mysqld
with commercial
memory-leakage detectors, so there should be no memory leaks.
All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.
The locking method MySQL uses for WRITE
locks works as follows:
The locking method MySQL uses for READ
locks works as follows:
When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.
This means that if you have many updates on a table, SELECT
statements will wait until there are no more updates.
To work around this for the case where you want to do many INSERT
and
SELECT
operations on a table, you can insert rows in a temporary
table and update the real table with the records from the temporary table
once in a while.
This can be done with the following code:
mysql> LOCK TABLES real_table WRITE, insert_table WRITE; mysql> insert into real_table select * from insert_table; mysql> delete from insert_table; mysql> UNLOCK TABLES;
You can use the LOW_PRIORITY
options with INSERT
if you
want to prioritize retrieval in some specific cases. See section 7.14 INSERT
syntax.
You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.
The table locking code in MySQL is deadlock free.
MySQL uses table locking (instead of row locking or column locking) to achieve a very high lock speed. For large tables, table locking is for most applications MUCH better than row locking, but there are of course some pitfalls.
Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update all others threads that want to access this particular table will wait until the update is ready.
As updates of databases normally are considered to be more important
than SELECT
, all statements that update a table have higher
priority than statements that retrieve information from a table. This
should ensure that updates are not 'starved' because one issues a lot of
heavy queries against a specific table.
One main problem with this is the following:
SELECT
that takes a long time to run.
INSERT
on a used table; This client
will wait until the SELECT
is finished..
SELECT
statement on the same table; As
INSERT
has higher priority than SELECT
, this SELECT
will wait for the INSERT
to finish. It will also wait for the first
SELECT
to finish!
Some possible solutions to this problem are:
SELECT
statements to run faster; You may have to create
some summary tables to do this.
mysqld
with --low-priority-updates
. This will give
all statements that update (modify) a table lower priority than a SELECT
statement. In this case the last SELECT
statement in the previous
scenario would execute before the INSERT
statement.
INSERT
,UPDATE
or DELETE
statement
lower priority with the LOW_PRIORITY
attribute.
SET SQL_LOW_PRIORITY_UPDATES=1
.
See section 7.25 SET OPTION
syntax.
SELECT
is very important with the
HIGH_PRIORITY
attribute. See section 7.12 SELECT
syntax.
INSERT
and SELECT
statements, the
DELAYED
attribute to INSERT
will probably solve your problems.
See section 7.14 INSERT
syntax.
SELECT
and DELETE
, the LIMIT
option to DELETE
may help. See section 7.11 DELETE
syntax.
One of the most basic optimization is to get your data (and indexes) to take as little space on the disk (and in memory) as possible. This can give huge improvements since disk reads are faster and normally less main memory will also be used. Indexing also takes less resources if done on smaller columns.
You can get better performance on a table and minimize storage space using the techniques listed below:
MEDIUMINT
is often better than INT
.
NOT NULL
if possible. It makes everything
faster and you save one bit per column. Note that if you really need
NULL
in your application you should definitely use it. Just avoid
haveing it on all columns by default.
VARCHAR
,
TEXT
or BLOB
columns), a fixed-size record format is
used. This is faster but unfortunately may waste some space.
See section 10.6 Choosing a table type.
Indexes are used to find find a row with a specific calue on one column fast. Without a index MySQL has to start with the first record and then read through the whole table until it find the relevent rows. The bigger the table the more this costs. If the table has a index for the colums in question MySQL can get fast a possition to seek to in the middle of the data file without having to look at all data. If a table have 1000 rows this is at least 100 times faster than reading sequentially. Note that is you need to access almost all 1000 rows it is faster to read sequentially since we when avoid disk seeks.
All MySQL indexes (PRIMARY
, UNIQUE
and INDEX
) are
stored in B-trees. Strings are automatically prefix- and end-space
compressed. See section 7.27 CREATE INDEX
syntax.
Indexes are used to:
WHERE
clause.
MAX()
or MIN()
value for a specific indexed
column.
ORDER BY key_part_1,key_part_2
). The
key is read in reverse order if all key parts are followed by DESC
.
Suppose you issue the following SELECT
statement:
mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;
If a multiple-column index exists on col1
and col2
, the
appropriate rows can be fetched directly. If separate single-column
indexes exist on col1
and col2
, the optimizer tries to
find the most restrictive index by deciding which index will find fewer
rows and using that index to fetch the rows.
If the table has a multiple-column index, any leftmost prefix of the
index can be used by the optimizer to find rows. For example, if you
have a three-column index on (col1,col2,col3)
, you have indexed
search capabilities on (col1)
, (col1,col2)
and
(col1,col2,col3)
.
MySQL can't use a partial index if the columns don't form a
leftmost prefix of the index. Suppose you have the SELECT
statements shown below:
mysql> SELECT * FROM tbl_name WHERE col1=val1; mysql> SELECT * FROM tbl_name WHERE col2=val2; mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;
If an index exists on (col1,col2,col3)
, only the first query
shown above uses the index. The second and third queries do involve
indexed columns, but (col2)
and (col2,col3)
are not
leftmost prefixes of (col1,col2,col3)
.
MySQL also uses indexes for LIKE
comparisons if the argument
to LIKE
is a constant string that doesn't start with a wildcard
character. For example, the following SELECT
statements use indexes:
mysql> select * from tbl_name where key_col LIKE "Patrick%"; mysql> select * from tbl_name where key_col LIKE "Pat%_ck%";
In the first statement, only rows with "Patrick" <= key_col <
"Patricl"
are considered. In the second statement, only rows with
"Pat" <= key_col < "Pau"
are considered.
The following SELECT
statements will not use indexes:
mysql> select * from tbl_name where key_col LIKE "%Patrick%"; mysql> select * from tbl_name where key_col LIKE other_col;
In the first statement, the LIKE
value begins with a wildcard character.
In the second statement, the LIKE
value is not a constant.
Searching using column_name IS NULL
will use indexes if column_name
is a index.
MySQL normally uses the index that finds least number of rows. An
index is used for columns that you compare with the following operators:
=
, >
, >=
, <
, <=
, BETWEEN
and a
LIKE
with a non-wildcard prefix like 'something%'
.
Any index that doesn't span all AND
levels in the WHERE
clause
is not used to optimize the query.
The following WHERE
clauses use indexes:
... WHERE index_part1=1 AND index_part2=2 ... WHERE index=1 OR A=10 AND index=2 /* index = 1 OR index = 2 */ ... WHERE index_part1='hello' AND index_part_3=5 /* optimized like "index_part1='hello'" */
These WHERE
clauses do NOT use indexes:
... WHERE index_part2=1 AND index_part3=2 /* index_part_1 is not used */ ... WHERE index=1 OR A=10 /* No index */ ... WHERE index_part1=1 OR index_part2=10 /* No index spans all rows */
First one thing that affect all querys. The more compex permission systen setup you have, the more overhead you get.
If you do not have any GRANT
statements done MySQL will optimize
the permission checking somewhat. So if you have a very high vokume it
may be worth the time to avoid grants. Otherwise more permission check
gives a larger overhead.
If your problem is with some explicit MySQL function, you can always time this in the MySQL client:
mysql> select benchmark(1000000,1+1); +------------------------+ | benchmark(1000000,1+1) | +------------------------+ | 0 | +------------------------+ 1 row in set (0.32 sec)
The above shows that MySQL can execute 1,000,000 +
expressions in 0.32 seconds on a PentiumII 400MHz
.
All MySQL functions should be very optimized, but there may be
some exceptions and the benchmark(loop_count,expression)
is a
great tool to find if this is a problem with your query.
In most cases you can estimate the performance by counting disk seeks.
For small tables you can usually find the row in 1 disk seek (as the
index is probably cached). For bigger tables, you can estimate that,
(using B++ tree indexes), you will need: log(row_count) /
log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) +
1
seeks to find a row.
In MySQL an index block is usually 1024 bytes and the data
pointer is usually 4 bytes, which gives for a 500,000 row table with a
index length of 3 (medium integer) gives you:
log(500,000)/log(1024/3*2/(3+4)) + 1
= 4 seeks.
As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3 (which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.
For writes you will however need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.
Note that the above doesn't mean that your application will slowly degenerate by N log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by N log N). To avoid this increase the index cache as the data grows. See section 10.2.3 Tuning server parameters.
SELECT
querys
In general, when you want to make a slow SELECT ... WHERE
faster,
the first thing to check is whether or not you can add an
index. See section 10.4 MySQL index use. All references between different tables
should usually be done with indexes. You can use the EXPLAIN
command to determine which indexes are used for a SELECT
.
See section 7.22 EXPLAIN
syntax (Get information about a SELECT
).
Some general tips:
myisamchk
--analyze
on a table after it has been loaded with relevant data. This
updates a value for each index that indicates the average number of rows
that have the same value. (For unique indexes, this is always 1, of
course.)
myisamchk
--sort-index --sort-records=1
(if you want to sort on index 1). If you
have a unique index from which you want to read all records in order
according to that index, this is a good way to make that faster. Note
however that this sorting isn't written optimally and will take a long
time for a large table!
WHERE
clauses
The where optimizes are put in the SELECT
part here since they
are mostly used there. But the same optimizations are used for there in
DELETE
and UPDATE
statements.
Also note that this section is incomplete. MySQL does many optimizations and we have not had time to document them all.
Some of the optimizations performed by MySQL are listed below:
((a AND b) AND c OR (((a AND b) AND (c AND d)))) -> (a AND b AND c) OR (a AND b AND c AND d)
(a<b AND b=c) AND a=5 -> b>5 AND b=c AND a=5
(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6) -> B=5 OR B=6
COUNT(*)
on a single table without a WHERE
is retrieved
directly from the table information. This is also done for any NOT NULL
expression when used with only one table.
SELECT
statements are impossible and returns no rows.
HAVING
is merged with WHERE
if you don't use GROUP
BY
or group functions (COUNT()
, MIN()
...)
WHERE
is constructed to get a fast
WHERE
evaluation for each sub join and also to skip records as
soon as possible.
WHERE
clause on a UNIQUE
index or a
PRIMARY KEY
, where all index parts are used with constant expressions.
mysql> SELECT * FROM t WHERE primary_key=1; mysql> SELECT * FROM t1,t2 WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
ORDER BY
and in GROUP
BY
come from the same table, then this table is preferred first when
joining.
ORDER BY
clause and a different GROUP BY
clause,
or if the ORDER BY
or GROUP BY
contains columns from tables other than the first table in the join
queue, a temporary table is created.
SQL_SMALL_RESULT
, MySQL will use an in-memory
temporary table.
DISTINCT
is converted to a GROUP BY
on all columns,
DISTINCT
combined with ORDER BY
will in many cases also need
a temporary table.
HAVING
clause
are skipped.
Some examples of queries that are very fast:
mysql> SELECT COUNT(*) FROM tbl_name; mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name; mysql> SELECT MAX(key_part2) FROM tbl_name WHERE key_part_1=constant; mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... LIMIT 10; mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;
The following queries are resolved using only the index tree (assuming the indexed columns are numeric):
mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val; mysql> SELECT COUNT(*) FROM tbl_name WHERE key_part1=val1 AND key_part2=val2; mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;
The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:
mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,...
LEFT JOIN
A LEFT JOIN B
is in MySQL implemented as follows
B
is set to be dependent on table A
.
A
is set to be dependent on all tables (except B
)
that are used in the LEFT JOIN
condition.
LEFT JOIN
conditions are moved to the WHERE
clause.
WHERE
optimzations are done.
A
that matches the WHERE
clause, but there
wasn't any row in B
that matched the LEFT JOIN
condition,
then an extra B
row is generated with all columns set to NULL
.
LEFT JOIN
to find rows that doesn't exist in some
table and you have the following test: column_name IS NULL
in the
WHERE
part, where column_name is a column that is declared as
NOT NULL
, then MySQL
will stop searching after more rows
(for a particular key combination) after it has found one row that
matches the LEFT JOIN
condition.
LIMIT
In some cases MySQL will handle the query differently when you are
using LIMIT #
and not using HAVING
:
LIMIT
, MySQL
will use indexes in some cases when it normally would prefer to do a
full table scan.
LIMIT #
with ORDER BY
, MySQL will end the
sorting as soon as it has found the first #
lines instead of sorting
the whole table.
LIMIT #
with DISTINCT
, MySQL will stop
as soon as it finds #
unique rows.
GROUP BY
can be resolved by reading the key in order
(or do a sort on the key) and then calculate summaries until the
key value changes. In this case LIMIT #
will not calculate any
unnecessary GROUP
's.
MySQL
has sent the first #
rows to the client, it
will abort the query.
LIMIT 0
will always quickly return an empty set. This is useful
to check the query and to get the column types of the result columns.
LIMIT #
to calculate how much
space is needed to resolve the query.
INSERT
querysThe time to insert a record consists of:
Where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently-running query).
The size of the table slows down the insertion of indexes by N log N (B-trees).
Some ways to speed up inserts:
INSERT
statements. This is much faster (many
times in some cases) than using separate INSERT
statements.
INSERT DELAYED
statement. See section 7.14 INSERT
syntax.
LOAD DATA INFILE
. This
is usually 20 times faster than using a lot of INSERT
statements.
See section 7.16 LOAD DATA INFILE
syntax.
LOAD DATA INFILE
even
faster when the table has many indexes. Use the following procedure:
CREATE TABLE
. For example using
mysql
or Perl-DBI.
FLUSH TABLES
. Or the shell command mysqladmin
flush-tables
myisamchk --keys-used=0 -rq /path/to/db/tbl_name
. This will
remove all usage of all indexes from the table.
LOAD DATA INFILE
. This will not
update any indexes and will therefore be very fast.
myisampack
and want to compress the table, run
myisampack
on it. See section 10.6.3 Compressed table characteristics.
myisamchk -r -q
/path/to/db/tbl_name
. This will create the index tree in memory before
writing it to disk. And that is much faster since it avoid lots of disk
seeks. The resulting index tree is also perfectly balanced.
FLUSH TABLES
. Or the shell command mysqladmin
flush-tables
LOAD DATA INFILE
in some future
version of MySQL.
mysql> LOCK TABLES a WRITE; mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33); mysql> INSERT INTO a VALUES (8,26),(6,29); mysql> UNLOCK TABLES;The main speed difference is that the index buffer is flushed to disk only once, after all
INSERT
statements have completed. Normally there would
be as many index buffer flushes as there are different INSERT
statements. Locking is not needed if you can insert all rows with a single
statement.
Locking will also lower the total time of multi-connection tests, but the
maximum wait time for some threads will go up (because they wait for
locks). For example:
thread 1 does 1000 inserts thread 2, 3, and 4 does 1 insert thread 5 does 1000 insertsIf you don't use locking, 2, 3 and 4 will finish before 1 and 5. If you use locking, 2, 3 and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster. As
INSERT
, UPDATE
and DELETE
operations are very
fast in MySQL, you will obtain better overall performance by
adding locks around everything that does more than about 5 inserts or
updates in a row. If you do very many inserts in a row, you could do a
LOCK TABLES
followed by a UNLOCK TABLES
once in a while
(about each 1000 rows) to allow other threads access to the table. This
would still result in a nice performance gain.
Of course, LOAD DATA INFILE
is much faster still for loading data.
To get some more speed for both LOAD DATA INFILE
and
INSERT
, enlarge the key buffer. See section 10.2.3 Tuning server parameters.
UPDATE
querys
Update queryes are optimized as a SELECT
query with the additiona
over head of a write. The speed of the write is dependent on the size of
the data that is being updated and the number of indexes that are
updated.
Also another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.
Not that with dynamic record format updating a record with to a longer
total length may split the record. So if you do this often it is very
important to OPTIMIZE TABLE
sometimes. See section 7.9 OPTIMIZE TABLE
syntax.
DELETE
querysThe time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 10.2.3 Tuning server parameters.
Its also much faster to remove all rows than to remove a big part of the rows from a table.
With MySQL you can currently (version 3.23.5) choose between four usable table formats from a speed point of view.
myisamchk
can easily figure out where each
row starts and ends. So it can usually reclaim all records except the
partially written one. Not that in MySQL all indexes can always be
reconstructed.
OPTIMIZE table
or myisamchk
to defragment a
table. If you have static data that you acess/change a lot in the same
table as some VARCHAR
or BLOB
columns, it might be a good
idea to move the dynamic columns to other tables just to avoid
fragmentation.
myisampack
tool.
SELECT tab1.a, tab3.a FROM tab1, tab2, tab3 WHERE tab1.a = tab2.a and tab2.a = tab3.a and tab2.c != 0;To speed this up we could create a temporary table with the join of tab2 and tab3 since that are looked up using the same column (tab1.a). Here is the command to create that table and the resulting select.
CREATE TEMPORARY TABLE test TYPE=HEAP SELECT tab2.a as a2, tab3.a as a3 FROM tab2, tab3 WHERE tab2.a = tab3.a and c = 0; SELECT tab1.a, test.a3 from tab1, test where tab1.a = test.a1; SELECT tab1.b, test.a3 from tab1, test where tab1.a = test.a1 and something;
VARCHAR
,
BLOB
or TEXT
columns.
CHAR
, NUMERIC
and DECIMAL
columns are space-padded
to the column width.
myisamchk
) unless a huge number of
records are deleted and you want to return free disk space to the operating
system.
VARCHAR
, BLOB
or TEXT
columns.
''
) for string columns, or zero for numeric columns (this isn't
the same as columns containing NULL
values). If a string column
has a length of zero after removal of trailing spaces, or a numeric
column has a value of zero, it is marked in the bit map and not saved to
disk. Non-empty strings are saved as a length byte plus the string
contents.
myisamchk
-r
from time to time to get better performance. Use myisamchk -ei
tbl_name
for some statistics.
3 + (number of columns + 7) / 8 + (number of char columns) + packed size of numeric columns + length of strings + (number of NULL columns + 7) / 8There is a penalty of 6 bytes for each link. A dynamic record is linked whenever an update causes an enlargement of the record. Each new link will be at least 20 bytes, so the next enlargement will probably go in the same link. If not, there will be another link. You may check how many links there are with
myisamchk -ed
. All links may be removed with myisamchk -r
.
myisampack
utility. All customers
with extended MySQL email support are entitled to a copy of
myisampack
for their internal usage.
myisampack
can read tables that
were compressed with myisampack
0
are stored using 1 bit.
BIGINT
column (8 bytes) may
be stored as a TINYINT
column (1 byte) if all values are in the range
0
to 255
.
ENUM
.
BLOB
or TEXT
columns.
myisamchk
.
MySQL can support different index types, but the normal type is
ISAM. This is a B-tree index and you can roughly calculate the size for the
index file as (key_length+4)*0.67
, summed over all keys. (This is for
the worst case when all keys are inserted in sorted order.)
String indexes are space compressed. If the first index part is a string, it
will also be prefix compressed. Space compression makes the index file
smaller if the string column has a lot of trailing space or is a VARCHAR
column that is not always used to the full length. Prefix compression helps
if there are many strings with an identical prefix.
HEAP tables only exists in memory so they are lost if mysqld
is
taken down or crashes. But since they are very fast they are
usefull as anyway.
The MySQL internal HEAP tables uses 100% dynamic hashing without overflow areas and don't have problems with delete.
You can only access things by equality using a index (usually by the
=
operator) whith a heap table.
The downside with HEAPS are:
ORDER BY
).
Unsorted tips for faster systems:
EXPLAIN
command. See section 7.22 EXPLAIN
syntax (Get information about a SELECT
).
SELECT
queries on tables that are updated a
lot. This is to avoid problems with table locking.
SELECT * from table where hash='calculated hash on col1 and col2'
and col_1='constant' and col_2='constant' and ..
VARCHAR
or BLOB
columns. You will get dynamic row length as soon as you
are using a single VARCHAR
or BLOB
columns. See section 9.4 MySQL table types.
UPDATE table set count=count+1 where index_column=constant
is very fast!
This is really important when you use databases like MySQL that
only has table locking (multiple readers / single writers). This will
also give better performance with most databases as the row locking
manager in this case will have less to do.
INSERT /*! DELAYED */
when you do not need to now when your
data is written. This speeds things up since many records can be written
with a single disk write.
INSERT /*! LOW_PRIORITY */
when you want your selects are
more important.
SELECT /*! HIGH_PRIORITY */
to get selects that jumps the
que. That is the select is done even if there is somebody waiting to do
a write.
INSERT
statement to store many rows with one
SQL command (many SQL servers supports this)
LOAD DATA INFILE
to load bigger amounts of data. This if
faster than normal inserts and will be even faster when myisamchk
is integrated in mysqld
.
AUTO_INCREMENT
columns to make unique values.
OPTIMIZE TABLE
once in a while to avoid fragmentation when
using dynamic table format.See section 7.9 OPTIMIZE TABLE
syntax.
HEAP
tables to get more speed when possibe. See section 9.4 MySQL table types.
name
instead of
customer_name
in the customer table). To make your names portable
to other SQL servers you should keep them shorter than 18 characters.
MyISAM
directly you could
get a speed increase of 2-5 times compared to using the SQL interface.
The be able to do this the data must however be on the same server as
the application and usually it should only be accessed by on processes
(because external file locking is real slow). One could eliminate the
above problems by introducing low level MyISAM
commands in the
MySQL server (this could be one easy way to get more
performance if needed). By carefully designing the database interface
it should be quite easy to support this types of optimisations.
You should definitely benchmark your application and database to find out where is the bottlenecks. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is 'good enough' you should at least make a 'plan', for each bottleneck, how to solve it if you someday 'really need it fix it'.
For some example portable becnchmark programs look at the MySQL benchmark suite. See section 11 The MySQL benchmark suite. You can take any program this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.
It is very common that some problems only occur then the system is very heavily loaded. And we have had many customer who contacts us then they have a (tested) system in production and have have got load problems. In every on these cases so far it has been problems with basic design (table scans are NOT good at high load) or OS/Library issues. Most of this would be a LOT easier to fix if the system where not already in production.
To avoid probles like this you should put some effort into benchmarking your whole appliction under the worst possible load!
MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We belive that the MySQL choice is better for a very wide range of modern systems.
Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will get a performance hit for every query that access more than one column. Since this degenerates so quickly when more that when one columns are accessed we believe that this model is not good for general purpose databases.
The more common case is there the index and data are stored together (like in Oracle/Sybase at all). In this case you will find the row information at the leaf page of the index. The good thing with this layout is that it in many cases (depends on how well the index is cached) saves a disk read. The bad things with this layout is:
Since MySQL uses extremely fast table locking (multiple readers / single writers) the biggest remaining problem is a mix of a steady stream of inserts and slow selects on the same table.
We belive that for a huge number of system the extreamy fast performance in other cases make this choice a win. This case is usually also possible to solve by having multiple copies of the table. But it takes more effort and hardware.
We are also working on some extension to solve this problem for some common application niches.
Since all SQL servers implement different parts of SQL it takes work to write portable SQL applications. For very simple selects/inserts it is very easy but the more you need the harder it gets. And if you want a application that is fast with many databases it becomes even harder!
To make a complex application portable you need to choose a number of SQL server that it should work with.
When you can use the MySQL crash-me program/web-page http://www.mysql.com/crash-me-choose.htmy to find functions, types and limits you can use with a selection of database servers. Crash-me now test a long way from everything possible but it still is v´comprehensive with about 450 things tested.
For example, you shouldn't have longer column names than 18 characters if you want to be able to use Informix or DB2.
Both the MySQL benchmarks and Crash-me programs are very database independent. By taking a look of how we have handled this, you can get a feeling of what you have to do to write your application database independent. The benchmark themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem.
See http://www.mysql.com/benchmark.html the results from this benchmark.
As you can see in these results all databases has some weak points. That is they have different design compromises that lead to different behavior.
If you strive for database independence you need to get a good feeling of each SQL servers bottlenecks. MySQL is VERY fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle on the other hand has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good in generating summary tables from log tables as in this case row locking is almost useless.
To get your application 'really database independent' you need to define a easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.
If you use some specific feature for some database (like the
REPLACE
command in MySQL), you should code a method for
the other SQL servers to implement the same feature (but slower). With
MySQL you can use the /*! */
syntax to add
MySQL specific keywords to a query. The code inside
/**/
will be treated as a comment (ignored) by most other SQL
servers.
If REAL high performance is more important than exactness, like in some web applications. A possibility is to create a application layer that caches all results to give you even higher performance. By just letting old results 'expire' after a while you can keep the cache reasonable fresh. This is quite nice in case of extremely high load, in which case you can dynamicly increase the cache to be bigger and set the expire timeout higher until things gets back to normal.
In this case the table creating information should contain information of the initial size of the cache and how often the table should normally be refreshed.
During MySQL initial development the features of MySQL where made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.
We get from all stores weekly summaries of all bonus card transactions and we are expected to provide useful information for the store owners to help them find how their advertisements campaigns are affecting their customers.
The data is quite huge (about 7 million summary transactions per month) and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.
We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros/script that generate summary tables grouped by different criterias (product group, customer id, store ...) from the transaction tables. The reports are web pages that are dynamicly generated by a small perl script that parses a web pages, executes the SQL statements in it and inserts the results. Now we would have used PHP or mod_perl instead but they where not available at that time.
For graphical data we wrote a simple tool in C
that can produce
gifs based on the result of a SQL query (with some processing of the
result). This is also dynamicly executed from the perl script that
parses the HTML
files.
In most cases a new report can simple by done by copying a existing script and modifying the SQL query in it. In some cases we will need to add more fields to an existing summary table or generate a new one, but this is also quite simply as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data).
We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.
We haven't had any problems handling this with quite modest Sun Ultra sparcstation (2x200 Mz). We recently upgrade one of our servers to a 2 CPU 400 Mz Ultra sparc and we are now planing to start handling transactions on the product level, which would mean a 10 fold increase of data. We think we can keep up with this by just adding more disk to our systems.
We are also experimenting with Intel-Linux to be able to get more cpu power cheaper. Now that we have the binary portable database format (new in 3.32) we will start to use this for some parts of the application.
Our initial feelings are that Linux will perform much better on low to medium load but Solaris will perform better when you start to get a a high load because of extrema disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This make the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.
This should contain a technical description of the MySQL
benchmark suite (and crash-me
) but that description is not
written yet. Currently, you should look at the code and results in the
`sql-bench' directory in the distribution (and of course on the web
page at http://www.mysql.com/crash-me-choose.htmy and (normally found in the `sql-bench' directory in the MySQL
distribution)).
It is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.
Note that this benchmark is single threaded so it measures the minimum time for the operations.
For example (run on the same NT 4.0 machine):
Seconds | Seconds | |
mysql | 367 | 249 |
mysql_odbc | 464 | |
db2_odbc | 1206 | |
informix_odbc | 121126 | |
ms-sql_odbc | 1634 | |
oracle_odbc | 20800 | |
solid_odbc | 877 | |
sybase_odbc | 17614 |
Seconds | Seconds | |
mysql | 381 | 206 |
mysql_odbc | 619 | |
db2_odbc | 3460 | |
informix_odbc | 2692 | |
ms-sql_odbc | 4012 | |
oracle_odbc | 11291 | |
solid_odbc | 1801 | |
sybase_odbc | 4802 |
In the above test MySQL was run with a 8M index cache.
Note that Oracle is not included since they asked to be removed. All Oracle benchmarks has to be passed by Oracle! We believe that makes Oracle benchmarks VERY biased since the above bechmarks are supposed to show that a standard installation can do for a single client.
crash-me
tries to determine what features a database supports and
what its capabilities and limitations are by actually running
queries. For example, it determines:
VARCHAR
column can be
All MySQL clients that communicate with the server using the
mysqlclient
library use the following environment variables:
Name | Description |
MYSQL_UNIX_PORT | The default socket; used for connections to localhost
|
MYSQL_TCP_PORT | The default TCP/IP port |
MYSQL_PWD | The default password |
MYSQL_DEBUG | Debug-trace options when debugging |
TMPDIR | The directory where temporary tables/files are created |
Use of MYSQL_PWD
is insecure.
See section 6.3 Connecting to the MySQL server.
The `mysql' client uses the file named in the MYSQL_HISTFILE
environment variable to save the command line history. The default value for
the history file is `$HOME/.mysql_history', where $HOME
is the
value of the HOME
environment variable.
All MySQL programs take many different options. However, every
MySQL program provides a --help
option that you can use
to get a full description of the program's different options. For example, try
mysql --help
.
You can override default options for all standard client programs with an option file. section 4.15.4 Option files.
The list below briefly describes the MySQL programs:
myisamchk
myisamchk
has many functions, it is described in its own
chapter. See section 13 Maintaining a MySQL installation.
make_binary_release
ftp.tcx.se
for the
convenience of other MySQL users.
msql2mysql
mSQL
programs to MySQL. It doesn't
handle all cases, but it gives a good start when converting.
mysql
mysql
is a simple SQL shell (with GNU readline
capabilities).
It supports interactive and non-interactive use. When used interactively,
query results are presented in an ASCII-table format. When used
non-interactively (e.g., as a filter), the result is presented in
tab-separated format. (The output format can be changed using command-line
options.) You can run scripts simply like this:
shell> mysql database < script.sql > output.tabIf you have problems due to insufficient memory in the client, use the
--quick
option! This forces mysql
to use
mysql_use_result()
rather than mysql_store_result()
to retrieve
the result set.
mysqlaccess
mysqladmin
mysqladmin
can also be used to retrieve version,
process and status information from the server.
See section 12.2 Administering a MySQL server.
mysqlbug
mysqld
mysqldump
mysqlimport
LOAD DATA
INFILE
. See section 12.4 Importing data from text files.
mysqlshow
mysql_install_db
replace
msql2mysql
, but that has more
general applicability as well. replace
changes strings in place in
files or on the standard input. Uses a finite state machine to match longer
strings first. Can be used to swap strings. For example, this command
swaps a
and b
in the given files:
shell> replace a b b a -- file1 file2 ...
safe_mysqld
mysqld
daemon with some safety features, such
as restarting the server when an error occurs and logging runtime information
to a log file.
Utility for performing administrative operations. The syntax is:
shell> mysqladmin [OPTIONS] command [command-option] command ...
You can get a list of the options your version of mysqladmin
supports
by executing mysqladmin --help
.
The current mysqladmin
supports the following commands:
create databasename | Create a new database. |
drop databasename | Delete a database and all its tables. |
extended-status | Gives an extended status message from the server. |
flush-hosts | Flush all cached hosts. |
flush-logs | Flush all logs. |
flush-tables | Flush all tables. |
flush-privileges | Reload grant tables (same as reload) |
kill id,id,... | Kill mysql threads. |
password | new-password Change old password to new-password |
ping | Check if mysqld is alive |
processlist | Show list of active threads in server |
reload | Reload grant tables |
refresh | Flush all tables and close and open logfiles |
shutdown | Take server down |
status | Gives a short status message from the server |
variables | Prints variables available |
version | Get version info from server |
All commands can be shortened to their unique prefix. For example:
shell> mysqladmin proc stat +----+-------+-----------+----+-------------+------+-------+------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+----+-------------+------+-------+------+ | 6 | monty | localhost | | Processlist | 0 | | | +----+-------+-----------+----+-------------+------+-------+------+ Uptime: 10077 Threads: 1 Questions: 9 Slow queries: 0 Opens: 6 Flush tables: 1 Open tables: 2 Memory in use: 1092K Max memory used: 1116K
The mysqladmin status
command result has the following columns:
Uptime | Number of seconds the MySQL server have been up |
Threads | Number of active threads (clients) |
Questions | Number of questions from clients since mysqld was started
|
Slow queries | Queries that has taken more than long_query_time seconds
|
Opens | How many tables mysqld has opened.
|
Flush tables | Number of flush ... , refresh and reload commands.
|
Open tables | Number of tables that are open now |
Memory in use | Memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug) |
Max memory used | Maximum memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug) |
Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server. The dump will contain SQL statements to create the table and/or populate the table.
shell> mysqldump [OPTIONS] database [tables]
If you don't give any tables, the whole database will be dumped.
You can get a list of the options your version of mysqldump
supports
by executing mysqldump --help
.
Note that if you run mysqldump
without --quick
or
--opt
, mysqldump
will load the whole result set into
memory before dumping the result. This will probably be a problem if
you are dumping a big database.
mysqldump
supports the following options:
--add-locks
LOCK TABLES
before and UNLOCK TABLE
after each table dump.
(To get faster inserts into MySQL).
--add-drop-table
drop table
before each create statement.
--allow-keywords
-c, --complete-insert
-C, --compress
--delayed
INSERT DELAYED
command.
-e, --extended-insert
INSERT
syntax. (Gives more compact and
faster inserts statements)
-#, --debug[=option_string]
--help
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--fields-terminated-by=...
-T
option and have the same
meaning as the corresponding clauses for LOAD DATA INFILE
.
See section 7.16 LOAD DATA INFILE
syntax.
-F, --flush-logs
-f, --force,
-h, --host=..
localhost
.
-l, --lock-tables.
-t, --no-create-info
CREATE TABLE
statment)
-d, --no-data
--opt
--quick --add-drop-table --add-locks --extended-insert
--lock-tables
. Should give you the fastest possible dump for reading
into a MySQL server.
-pyour_pass, --password[=your_pass]
mysqldump
solicits the password from the terminal.
-P port_num, --port=port_num
localhost
, for which Unix sockets are
used.)
-q, --quick
mysql_use_result()
to do this.
-S /path/to/socket, --socket=/path/to/socket
localhost
(which is the
default host).
-T, --tab=path-to-some-directory
table_name.sql
file, that conntains the SQL CREATE commands,
and a table_name.txt
file, that contains the data, for each give table.
NOTE: This only works if mysqldump
is run on the same
machine as the mysqld
daemon. The format of the .txt
file
is made according to the --fields-xxx
and --lines--xxx
options.
-u user_name, --user=user_name
-O var=option, --set-variable var=option
-v, --verbose
-V, --version
-w, --where='where-condition'
"--where=user='jimf'" "-wuserid>1" "-wuserid<1"
The most normal use of mysqldump
is probably for making a backup of
whole database:
mysqldump --opt database > backup-file.sql
But it's also very useful to populate another MySQL server with information from a database:
mysqldump --opt database | mysql --host=remote-host -C database
mysqlimport
provides a command line interface to the LOAD DATA
INFILE
SQL statement. Most options to mysqlimport
correspond
directly to the same options to LOAD DATA INFILE
.
See section 7.16 LOAD DATA INFILE
syntax.
mysqlimport
is invoked like this:
shell> mysqlimport [options] filename ...
For each text file named on the command line,
mysqlimport
strips any extension from the filename and uses the result
to determine which table to import the file's contents into. For example,
files named `patient.txt', `patient.text' and `patient' would
all be imported into a table named patient
.
mysqlimport
supports the following options:
-C, --compress
-#, --debug[=option_string]
-d, --delete
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--fields-terminated-by=...
LOAD DATA INFILE
. See section 7.16 LOAD DATA INFILE
syntax.
-f, --force
--force
,
mysqlimport
exits if a table doesn't exist.
--help
-h host_name, --host=host_name
localhost
.
-i, --ignore
--replace
option.
-l, --lock-tables
-L, --local
localhost
(which is the default host).
-pyour_pass, --password[=your_pass]
mysqlimport
solicits the password from the terminal.
-P port_num, --port=port_num
localhost
, for which Unix sockets are
used.)
-r, --replace
--replace
and --ignore
options control handling of input
records that duplicate existing records on unique key values. If you specify
--replace
, new rows replace existing rows that have the same unique key
value. If you specify --ignore
, input rows that duplicate an existing
row on a unique key value are skipped. If you don't specify either option, an
error occurs when a duplicate key value is found, and the rest of the text
file is ignored.
-s, --silent
-S /path/to/socket, --socket=/path/to/socket
localhost
(which is the
default host).
-u user_name, --user=user_name
-v, --verbose
-V, --version
myisampack
is used to compress MyISAM tables and pack_isam
is used to compress ISAM tables. Since ISAM tables are deprecated we
will only discuss myisampack
here.
myisampack
are a extra utilitie that you get when you order more
than 10 licenses or extended support. Since these are distributed only
in binary form, they are available only on some platforms.
In the following we only talk about pack_isam
, but everything
holds also for myisampack
.
myisampack
works by compressing each column in the table separately.
The information needed to decompress columns is read into memory when the
table is opened. This results in much better performance when accessing
individual records, since you only have to uncompress exactly one record, not
a much larger disk block like when using Stacker on MS-DOS.
Usually, myisampack
packs the data file 40%-70%.
MySQL uses memory mapping (mmap()
) on compressed tables and
falls back to normal read/write file usage if mmap()
doesn't work.
There are currently two limitations with myisampack
:
myisampack
can also pack BLOB
or TEXT
columns. The
older pack_isam
could not do this.
Fixing these limitations is on our TODO list but with low priority.
myisampack
is invoked like this:
shell> myisampack [options] filename ...
Each filename should be the name of an index (`.MYI') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.MYI' extension.
myisampack
supports the following options:
-b, --backup
tbl_name.OLD
.
-#, --debug=debug_options
debug_options
string often is
'd:t:o,filename'
.
-f, --force
myisampack
creates a temporary file named `tbl_name.TMD'
while it compresses the table. If you kill myisampack
, the `.TMD'
file may not be deleted. Normally, myisampack
exits with an error if
it finds that `tbl_name.TMD' exists. With --force
,
myisampack
packs the table anyway.
-?, --help
-j big_tbl_name, --join=big_tbl_name
big_tbl_name
. All tables that are to be combined
MUST be identical (same column names and types, same indexes, etc.)
-p #, --packlength=#
myisampack
stores all rows with length pointers of 1, 2 or 3
bytes. In most normal cases, myisampack
can determine the right length
value before it begins packing the file, but it may notice during the packing
process that it could have used a shorter length. In this case,
myisampack
will print a note that the next time you pack the same file,
you could use a shorter record length.)
-s, --silent
-t, --test
-T dir_name, --tmp_dir=dir_name
-v, --verbose
-V, --version
-w, --wait
mysqld
server was
invoked with the --skip-locking
option, it is not a good idea to
invoke myisampack
if the table might be updated during the
packing process.
The sequence of commands shown below illustrates a typical table compression session:
shell> ls -l station.* -rw-rw-r-- 1 monty my 994128 Apr 17 19:00 station.MYD -rw-rw-r-- 1 monty my 53248 Apr 17 19:00 station.MYI -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> myisamchk -dvv station MyISAM file: station Isam-version: 2 Creation time: 1996-03-13 10:08:58 Recover time: 1997-02-02 3:06:43 Data records: 1192 Deleted blocks: 0 Datafile: Parts: 1192 Deleted data: 0 Datafile pointer (bytes): 2 Keyfile pointer (bytes): 2 Max datafile length: 54657023 Max keyfile length: 33554431 Recordlength: 834 Record format: Fixed length table description: Key Start Len Index Type Root Blocksize Rec/key 1 2 4 unique unsigned long 1024 1024 1 2 32 30 multip. text 10240 1024 1 Field Start Length Type 1 1 1 2 2 4 3 6 4 4 10 1 5 11 20 6 31 1 7 32 30 8 62 35 9 97 35 10 132 35 11 167 4 12 171 16 13 187 35 14 222 4 15 226 16 16 242 20 17 262 20 18 282 20 19 302 30 20 332 4 21 336 4 22 340 1 23 341 8 24 349 8 25 357 8 26 365 2 27 367 2 28 369 4 29 373 4 30 377 1 31 378 2 32 380 8 33 388 4 34 392 4 35 396 4 36 400 4 37 404 1 38 405 4 39 409 4 40 413 4 41 417 4 42 421 4 43 425 4 44 429 20 45 449 30 46 479 1 47 480 1 48 481 79 49 560 79 50 639 79 51 718 79 52 797 8 53 805 1 54 806 1 55 807 20 56 827 4 57 831 4 shell> myisampack station.MYI Compressing station.MYI: (1192 records) - Calculating statistics normal: 20 empty-space: 16 empty-zero: 12 empty-fill: 11 pre-space: 0 end-space: 12 table-lookups: 5 zero: 7 Original trees: 57 After join: 17 - Compressing file 87.14% shell> ls -l station.* -rw-rw-r-- 1 monty my 127874 Apr 17 19:00 station.MYD -rw-rw-r-- 1 monty my 55296 Apr 17 19:04 station.MYI -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> myisamchk -dvv station MyISAM file: station Isam-version: 2 Creation time: 1996-03-13 10:08:58 Recover time: 1997-04-17 19:04:26 Data records: 1192 Deleted blocks: 0 Datafile: Parts: 1192 Deleted data: 0 Datafilepointer (bytes): 3 Keyfile pointer (bytes): 1 Max datafile length: 16777215 Max keyfile length: 131071 Recordlength: 834 Record format: Compressed table description: Key Start Len Index Type Root Blocksize Rec/key 1 2 4 unique unsigned long 10240 1024 1 2 32 30 multip. text 54272 1024 1 Field Start Length Type Huff tree Bits 1 1 1 constant 1 0 2 2 4 zerofill(1) 2 9 3 6 4 no zeros, zerofill(1) 2 9 4 10 1 3 9 5 11 20 table-lookup 4 0 6 31 1 3 9 7 32 30 no endspace, not_always 5 9 8 62 35 no endspace, not_always, no empty 6 9 9 97 35 no empty 7 9 10 132 35 no endspace, not_always, no empty 6 9 11 167 4 zerofill(1) 2 9 12 171 16 no endspace, not_always, no empty 5 9 13 187 35 no endspace, not_always, no empty 6 9 14 222 4 zerofill(1) 2 9 15 226 16 no endspace, not_always, no empty 5 9 16 242 20 no endspace, not_always 8 9 17 262 20 no endspace, no empty 8 9 18 282 20 no endspace, no empty 5 9 19 302 30 no endspace, no empty 6 9 20 332 4 always zero 2 9 21 336 4 always zero 2 9 22 340 1 3 9 23 341 8 table-lookup 9 0 24 349 8 table-lookup 10 0 25 357 8 always zero 2 9 26 365 2 2 9 27 367 2 no zeros, zerofill(1) 2 9 28 369 4 no zeros, zerofill(1) 2 9 29 373 4 table-lookup 11 0 30 377 1 3 9 31 378 2 no zeros, zerofill(1) 2 9 32 380 8 no zeros 2 9 33 388 4 always zero 2 9 34 392 4 table-lookup 12 0 35 396 4 no zeros, zerofill(1) 13 9 36 400 4 no zeros, zerofill(1) 2 9 37 404 1 2 9 38 405 4 no zeros 2 9 39 409 4 always zero 2 9 40 413 4 no zeros 2 9 41 417 4 always zero 2 9 42 421 4 no zeros 2 9 43 425 4 always zero 2 9 44 429 20 no empty 3 9 45 449 30 no empty 3 9 46 479 1 14 4 47 480 1 14 4 48 481 79 no endspace, no empty 15 9 49 560 79 no empty 2 9 50 639 79 no empty 2 9 51 718 79 no endspace 16 9 52 797 8 no empty 2 9 53 805 1 17 1 54 806 1 3 9 55 807 20 no empty 3 9 56 827 4 no zeros, zerofill(2) 2 9 57 831 4 no zeros, zerofill(1) 2 9
The information printed by myisampack
is described below:
normal
empty-space
empty-zero
empty-fill
INTEGER
column may be changed to MEDIUMINT
).
pre-space
end-space
table-lookup
ENUM
before Huffman compression.
zero
Original trees
After join
After a table has been compressed, myisamchk -dvv
prints additional
information about each field:
Type
constant
no endspace
no endspace, not_always
no endspace, no empty
table-lookup
ENUM
.
zerofill(n)
n
bytes in the value are always 0 and are not
stored.
no zeros
always zero
Huff tree
Bits
myisamchk
for table maintenance and crash recovery
To check/repair MyISAM tables (.MYI
and .MYD
) you should
use the myisamchk
utility. To check/repair ISAM tables
(.ISM
and .ISD
) you should use the isamchk
utility. See section 9.4 MySQL table types.
In the following text we will talk about myisamchk
but everything
also applies to the old isamchk
.
You can use the myisamchk
utility to get information about your database
tables, check and repair them or optimize them. The following sections
describe how to invoke myisamchk
(including a description of its
options), how to set up a table maintenance schedule, and how to use
myisamchk
to perform its various functions.
If you run mysqld
with --skip-locking
(which is the default on
some systems, like Linux), you can't reliably use myisamchk
to
check a table when mysqld
is using the same table. If you
can be sure that no one is accessing the tables through mysqld
while you run myisamchk
, you only have to do mysqladmin
flush-tables
before you start checking the tables. If you can't
guarantee the above, then you must take down mysqld
while you
check the tables. If you run myisamchk
while mysqld
is updating
the tables, you may get a warning that a table is corrupt even if it
isn't.
If you are not using --skip-locking
, you can use myisamchk
to check tables at any time. While you do this, all clients that try
to update the table will wait until myisamchk
is ready before
continuing.
If you use myisamchk
to repair or optimize tables, you
MUST always ensure that the mysqld
server is not using
the table (this also applies if you are using --skip-locking
).
If you don't take down mysqld
you should at least do a
mysqladmin flush-tables
before you run myisamchk
.
You can in most cases also use the command OPTIMIZE TABLES
to
optimize and repair tables, but this is not as fast or reliable (in case
of real fatal errors) as myisamchk
. On the other hand,
OPTIMIZE TABLE
is easier to use and you don't have to worry about
flushing tables.
See section 7.9 OPTIMIZE TABLE
syntax.
myisamchk
invocation syntax
myisamchk
is invoked like this:
shell> myisamchk [options] tbl_name
The options
specify what you want myisamchk
to do. They are
described below. (You can also get a list of options by invoking
myisamchk --help
.) With no options, myisamchk
simply checks your
table. To get more information or to tell myisamchk
to take corrective
action, specify options as described below and in the following sections.
tbl_name
is the database table you want to check. If you run
myisamchk
somewhere other than in the database directory, you must
specify the path to the file, since myisamchk
has no idea where your
database is located. Actually, myisamchk
doesn't care whether or not
the files you are working on are located in a database directory; you can
copy the files that correspond to a database table into another location and
perform recovery operations on them there.
You can name several tables on the myisamchk
command line if you
wish. You can also specify a name as an index file
name (with the `.MYI' suffix), which allows you to specify all
tables in a directory by using the pattern `*.MYI'.
For example, if you are in a database directory, you can check all the
tables in the directory like this:
shell> myisamchk *.MYI
If you are not in the database directory, you can check all the tables there by specifying the path to the directory:
shell> myisamchk /path/to/database_dir/*.MYI
You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory:
shell> myisamchk /path/to/datadir/*/*.MYI
myisamchk
supports the following options:
-a, --analyze
-#, --debug=debug_options
debug_options
string often is
'd:t:o,filename'
.
-d, --description
-e, --extend-check
myisamchk
should find all errors even without this option.
-f, --force
-f
when checking tables (running myisamchk
without -r
), myisamchk
will automatically restart with -r
on any table for which an error occurs during checking.
--help
-i, --information
-k #, --keys-used=#
-r
. Tell the ISAM table handler to update only the first
#
indexes. Higher-numbered indexes are deactivated. This can be used
to get faster inserts! Deactivated indexes can be reactivated by using
myisamchk -r
.
-l, --no-symlinks
myisamchk
repairs the table a symlink points at.
-q, --quick
-r
to get a faster repair. Normally, the original data file
isn't touched; you can specify a second -q
to force
the original data file to be used.
-r, --recover
-o, --safe-recover
-r
, but can
handle a couple of cases that -r
cannot handle.
-O var=option, --set-variable var=option
-s, --silent
-s
twice (-ss
) to make myisamchk
very silent.
-S, --sort-index
-R index_num, --sort-records=index_num
SELECT
and ORDER BY
operations on
this index. (It may be VERY slow to do a sort the first time!)
To find out a table's index numbers, use SHOW INDEX
, which shows a
table's indexes in the same order that myisamchk
sees them. Indexes are
numbered beginning with 1.
-u, --unpack
myisampack
.
-v, --verbose
-d
and
-e
. Use -v
multiple times (-vv
, -vvv
) for more
verbosity!
-V, --version
myisamchk
version and exit.
-w, --wait
Possible variables for the --set-variable
(-O
) option are:
key_buffer_size current value: 16776192 read_buffer_size current value: 262136 write_buffer_size current value: 262136 sort_buffer_size current value: 2097144 sort_key_blocks current value: 16 decode_bits current value: 9
myisamchk
memory usage
Memory allocation is important when you run myisamchk
. myisamchk
uses no more memory than you specify with the -O
options. If you are
going to use myisamchk
on very large files, you should first decide how
much memory you want it to use. The default is to use only about 3M to fix
things. By using larger values, you can get myisamchk
to operate
faster. For example, if you have more than 32M RAM, you could use options
such as these (in addition to any other options you might specify):
shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...
Using -O sort=16M
should
probably be enough for most cases.
Be aware that myisamchk
uses temporary files in TMPDIR
. If
TMPDIR
points to a memory file system, you may easily get out of
memory errors. If this happens, set TMPDIR
to point at some directory
with more space and restart myisamchk
It is a good idea to perform table checks on a regular basis rather than
waiting for problems to occur. For maintenance purposes, you can use
myisamchk -s
to check tables. The -s
option causes
myisamchk
to run in silent mode, printing messages only when errors
occur.
It's a good idea to check tables when the server starts up.
For example, whenever the machine has done a reboot in the middle of an
update, you usually need to check all the tables that could have been
affected. (This is an ``expected crashed table''.) You could add a test to
safe_mysqld
that runs myisamchk
to check all tables that have
been modified during the last 24 hours if there is an old `.pid'
(process ID) file left after a reboot. (The `.pid' file is created by
mysqld
when it starts up and removed when it terminates normally. The
presence of a `.pid' file at system startup time indicates that
mysqld
terminated abnormally.)
An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.
You should also check your tables regularly during normal system operation.
At TcX, we run a cron
job to check all our important tables once a week,
using a line like this in a `crontab' file:
35 0 * * 0 /path/to/myisamchk -s /path/to/datadir/*/*.MYI
This prints out information about crashed tables so we can examine and repair them when needed.
As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.
We recommend that to start with, you execute myisamchk -s
each
night on all tables that have been updated during the last 24 hours,
until you come to trust MySQL as much as we do.
To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later.
myisamchk -d tbl_name
myisamchk
in ``describe mode'' to produce a description of your
table. If you start the MySQL server using the --skip-locking
option, myisamchk
may report an error for a table that is updated while
it runs. However, since myisamchk
doesn't change the table in describe
mode, there isn't any risk of destroying data.
myisamchk -d -v tbl_name
myisamchk
is doing, add -v
to tell it to run in verbose mode.
myisamchk -eis tbl_name
myisamchk -eiv tbl_name
-eis
, but tells you what is being done.
Example of myisamchk -d
output:
MyISAM file: company.MYI Record format: Fixed length Data records: 1403698 Deleted blocks: 0 Recordlength: 226 table description: Key Start Len Index Type 1 2 8 unique double 2 15 10 multip. text packed stripped 3 219 8 multip. double 4 63 10 multip. text packed stripped 5 167 2 multip. unsigned short 6 177 4 multip. unsigned long 7 155 4 multip. text 8 138 4 multip. unsigned long 9 177 4 multip. unsigned long 193 1 text
Example of myisamchk -d -v
output:
MyISAM file: company Record format: Fixed length File-version: 1 Creation time: 1999-10-30 12:12:51 Recover time: 1999-10-31 19:13:01 Status: checked Data records: 1403698 Deleted blocks: 0 Datafile parts: 1403698 Deleted data: 0 Datafilepointer (bytes): 3 Keyfile pointer (bytes): 3 Max datafile length: 3791650815 Max keyfile length: 4294967294 Recordlength: 226 table description: Key Start Len Index Type Rec/key Root Blocksize 1 2 8 unique double 1 15845376 1024 2 15 10 multip. text packed stripped 2 25062400 1024 3 219 8 multip. double 73 40907776 1024 4 63 10 multip. text packed stripped 5 48097280 1024 5 167 2 multip. unsigned short 4840 55200768 1024 6 177 4 multip. unsigned long 1346 65145856 1024 7 155 4 multip. text 4995 75090944 1024 8 138 4 multip. unsigned long 87 85036032 1024 9 177 4 multip. unsigned long 178 96481280 1024 193 1 text
Example of myisamchk -eis
output:
Checking MyISAM file: company Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 98% Packed: 17% Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1626.51, System time 232.36 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 627, Swaps 0 Blocks in 0 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 639, Involuntary context switches 28966
Example of myisamchk -eiv
output:
Checking MyISAM file: company Data records: 1403698 Deleted blocks: 0 - check file-size - check delete-chain block_size 1024: index 1: index 2: index 3: index 4: index 5: index 6: index 7: index 8: index 9: No recordlinks - check index reference - check data record references index: 1 Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 2 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 - check data record references index: 3 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 - check data record references index: 5 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 6 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 7 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 8 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 9 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 9% Packed: 17% - check records and index references [LOTS OF ROW NUMBERS DELETED] Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1639.63, System time 251.61 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0 Blocks in 4 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 10604, Involuntary context switches 122798
Here are the sizes of the data and index files for the table used in the preceding examples:
-rw-rw-r-- 1 monty tcx 317235748 Jan 12 17:30 company.MYD -rw-rw-r-- 1 davida tcx 96482304 Jan 12 18:35 company.MYM
Explanations for the types of information myisamchk
produces are
given below. The ``keyfile'' is the index file. ``Record'' and ``row''
are synonymous.
ISAM file
Isam-version
Creation time
Recover time
Data records
Deleted blocks
Datafile: Parts
Data
records
.
Deleted data
Datafile pointer
Keyfile pointer
Max datafile length
.MYD
file) can become, in bytes.
Max keyfile length
.MYI
file) can become, in bytes.
Recordlength
Record format
Fixed length
.
Other possible values are Compressed
and Packed
.
table description
Key
Start
Len
Index
unique
or multip.
(multiple). Indicates whether or not one value
can exist multiple times in this index.
Type
packed
, stripped
or empty
.
Root
Blocksize
Rec/key
myisamchk -a
. If this is not updated at all, a default
value of 30 is given.
Keyblocks used
myisamchk
, the values are very
high (very near the theoretical maximum).
Packed
CHAR
/VARCHAR
/DECIMAL
keys. For long strings like
names, this can significantly reduce the space used. In the third example
above, the 4th key is 10 characters long and a 60% reduction in space is
achieved.
Max levels
Records
M.recordlength
Packed
Packed
value indicates the percentage savings achieved by doing this.
Recordspace used
Empty space
Blocks/Record
myisamchk
.
See section 13.4.3 Table optimization.
Recordblocks
Deleteblocks
Recorddata
Deleted data
Lost space
Linkdata
Linkdata
is the sum of the amount of
storage used by all such pointers.
If a table has been compressed with myisampack
, myisamchk
-d
prints additional information about each table column. See
section 12.5 The MySQL compressed read-only table generator, for an example of this
information and a description of what it means.
myisamchk
for crash recoveryThe file format that MySQL uses to store data has been extensively tested, but there are always external circumstances that may cause database tables to become corrupted:
mysqld
process being killed in the middle of a write
This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted a lot you should try to find the reason for this! See section G.1 Debugging a MySQL server.
When performing crash recovery, it is important to understand that each table
tbl_name
in a database corresponds to three files in the database
directory:
File | Purpose |
`tbl_name.frm' | Table definition (form) file |
`tbl_name.MYD' | Data file |
`tbl_name.MYI' | Index file |
Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.
myisamchk
works by creating a copy of the `.MYD' (data) file
row by row. It ends the repair stage by removing the old `.MYD'
file and renaming the new file to the original file name. If you use
--quick
, myisamchk
does not create a temporary `.MYD'
file, but instead assumes that the `.MYD' file is correct and only
generates a new index file without touching the `.MYD' file. This
is safe, because myisamchk
automatically detects if the
`.MYD' file is corrupt and aborts the repair in this case. You can
also give two --quick
options to myisamchk
. In this case,
myisamchk
does not abort on some errors (like duplicate key) but
instead tries to resolve them by modifying the `.MYD'
file. Normally the use of two --quick
options is useful only if
you have too little free disk space to perform a normal repair. In this
case you should at least make a backup before running myisamchk
.
To check a table, use the following commands:
myisamchk tbl_name
myisamchk
without options or
with either the -s
or --silent
option.
myisamchk -e tbl_name
-e
means
``extended check''). It does a check-read of every key for each row to verify
that they indeed point to the correct row. This may take a LONG time on a
big table with many keys. myisamchk
will normally stop after the first
error it finds. If you want to obtain more information, you can add the
--verbose
(-v
) option. This causes myisamchk
to keep
going, up through a maximum of 20 errors. In normal usage, a simple
myisamchk
(with no arguments other than the table name) is sufficient.
myisamchk -e -i tbl_name
-i
option tells myisamchk
to
print some informational statistics, too.
The symptoms of a corrupted table are usually that queries abort unexpectedly and that you observe errors such as these:
In these cases, you must repair your tables. myisamchk
can usually detect and fix most things that go wrong.
The repair process involves up to four stages, described below. Before you
begin, you should cd
to the database directory and check the
permissions of the table files. Make sure they are readable by the Unix user
that mysqld
runs as (and to you, since you need to access the files
you are checking). If it turns out you need to modify files, they must also
be writable by you.
Stage 1: Checking your tables
Run myisamchk *.MYI
or (myisamchk -e *.MYI
if you have more time).
Use the -s
(silent) option to suppress unnecessary information.
You have to repair only those tables for which myisamchk
announces an
error. For such tables, proceed to Stage 2.
If you get weird errors when checking (such as out of
memory
errors), or if myisamchk
crashes, go to Stage 3.
Stage 2: Easy safe repair
First, try myisamchk -r -q tbl_name
(-r -q
means ``quick recovery
mode''). This will attempt to repair the index file without touching the data
file. If the data file contains everything that it should and the delete
links point at the correct locations within the data file, this should work
and the table is fixed. Start repairing the next table. Otherwise, use the
following procedure:
myisamchk -r tbl_name
(-r
means ``recovery mode''). This will
remove incorrect records and deleted records from the data file and
reconstruct the index file.
myisamchk --safe-recover tbl_name
.
Safe recovery mode uses an old recovery method that handles a few cases that
regular recovery mode doesn't (but is slower).
If you get weird errors when repairing (such as out of
memory
errors), or if myisamchk
crashes, go to Stage 3.
Stage 3: Difficult repair
You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:
shell> mysql db_name mysql> DELETE FROM tbl_name; mysql> quit
Go back to Stage 2. myisamchk -r -q
should work now. (This shouldn't
be an endless loop).
Stage 4: Very difficult repair
You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created.
myisamchk -r
.
To coalesce fragmented records and eliminate wasted space resulting from
deleting or updating records, run myisamchk
in recovery mode:
shell> myisamchk -r tbl_name
You can optimize a table in the same way using the SQL OPTIMIZE TABLE
statement. OPTIMIZE TABLE
is easier, but myisamchk
is faster.
There is also no possibility of unwanted interaction between a utility
and the server, because the server does all the work when you use
OPTIMIZE TABLE
.
myisamchk
also has a number of other options you can use to improve
the performance of a table:
-S, --sort-index
-R index_num, --sort-records=index_num
-a, --analyze
For a full description of the option see See section 13.1.1 myisamchk
invocation syntax.
When using MySQL with log files, you will from time to time want to remove/backup old log files and tell MySQL to start logging on new files. See section 9.2 The update log.
One a Linux (Redhat
) installation, you can use the
mysql-log-rotate
script for this. If you installed MySQL
from an RPM distribution, the script should have been installed
automatically.
On other systems you must install a short script yourself that you
start from cron
to handle log files.
You can force MySQL to start using new log files by using
mysqladmin flush-logs
or by using the SQL command FLUSH LOGS
.
If you are using MySQL 3.21 you must use mysqladmin refresh
.
The above command does the following:
--log
) is used, closes and reopens the log file.
(`mysql.log' as default).
--log-update
) is used, closes the update log and
opens a new log file with a higher sequence number.
If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like:
shell> cd mysql-data-directory shell> mv mysql.log mysql.old shell> mysqladmin flush-tables
and then take a backup and remove `mysql.old'.
There are two ways to add new functions to MySQL:
CREATE FUNCTION
and DROP FUNCTION
statements.
See section 7.30 CREATE FUNCTION/DROP FUNCTION
syntax.
mysqld
server and become
available on a permanent basis.
Each method has advantages and disadvantages:
Whichever method you use to add new functions, they may be used just like
native functions such as ABS()
or SOUNDEX()
.
For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. The MySQL source distribution includes a file `sql/udf_example.cc' that defines 5 new functions. Consult this file to see how UDF calling conventions work.
For each function that you want to use in SQL statements, you should define
corresponding C (or C++) functions. In the discussion below, the name
``xxx'' is used for an example function name. To distinquish between SQL and
C/C++ usage, XXX()
(uppercase) indicates a SQL function call, and
xxx()
(lowercase) indicates a C/C++ function call.
The C/C++ functions that you write to implement the inferface for
XXX()
are:
xxx()
(required)
SQL type | C/C++ type |
STRING | char *
|
INTEGER | long long
|
REAL | double
|
xxx_init()
(optional)
xxx()
. It can be used to:
XXX()
REAL
functions) the maximum number of decimals
NULL
xxx_deinit()
(optional)
xxx()
. It should deallocate any
memory allocated by the initialization function.
When a SQL statement invokes XXX()
, MySQL calls the
initialization function xxx_init()
to let it perform any required
setup, such as argument checking or memory allocation. If xxx_init()
returns an error, the SQL statement is aborted with an error message and the
main and deinitialization functions are not called. Otherwise, the main
function xxx()
is called once for each row. After all rows have been
processed, the deinitialization function xxx_deinit()
is called so it
can perform any required cleanup.
All functions must be thread-safe (not just the main function,
but the initialization and deinitialization functions as well). This means
that you are not allowed to allocate any global or static variables that
change! If you need memory, you should allocate it in xxx_init()
and free it in xxx_deinit()
.
The main function should be declared as shown below. Note that the return
type and parameters differ, depending on whether you will declare the SQL
function XXX()
to return STRING
, INTEGER
or REAL
in the CREATE FUNCTION
statement:
For STRING
functions:
char *xxx(UDF_INIT *initid, UDF_ARGS *args, char *result, unsigned long *length, char *is_null, char *error);
For INTEGER
functions:
long long xxx(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
For REAL
functions:
double xxx(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
The initialization and deinitialization functions are declared like this:
my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message); void xxx_deinit(UDF_INIT *initid);
The initid
parameter is passed to all three functions. It points to a
UDF_INIT
structure that is used to communicate information between
functions. The UDF_INIT
structure members are listed below. The
initialization function should fill in any members that it wishes to change.
(To use the default for a member, leave it unchanged.)
my_bool maybe_null
xxx_init()
should set maybe_null
to 1
if xxx()
can return NULL
. The default value is 1
if any of the
arguments are declared maybe_null
.
unsigned int decimals
1.34
, 1.345
and 1.3
, the default would be 3,
since 1.345
has 3 decimals.
unsigned int max_length
initid->decimals
. (For numeric functions, the length
includes any sign or decimal point characters.)
char *ptr
initid->ptr
to communicate allocated memory
between functions. In xxx_init()
, allocate the memory and assign it
to this pointer:
initid->ptr = allocated_memory;In
xxx()
and xxx_deinit()
, refer to initid->ptr
to use
or deallocate the memory.
The args
parameter points to a UDF_ARGS
structure which has the
members listed below:
unsigned int arg_count
if (args->arg_count != 2) { strcpy(message,"XXX() requires two arguments"); return 1; }
enum Item_result *arg_type
STRING_RESULT
, INT_RESULT
and REAL_RESULT
.
To make sure that arguments are of a given type and return an
error if they are not, check the arg_type
array in the initialization
function. For example:
if (args->arg_type[0] != STRING_RESULT && args->arg_type[1] != INT_RESULT) { strcpy(message,"XXX() requires a string and an integer"); return 1; }As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the
arg_type
elements to the types you want. This causes MySQL to coerce
arguments to those types for each call to xxx()
. For example, to
specify coercion of the first two arguments to string and integer, do this in
xxx_init()
:
args->arg_type[0] = STRING_RESULT; args->arg_type[1] = INT_RESULT;
char **args
args->args
communicates information to the initialization function
about the general nature of the arguments your function was called with. For a
constant argument i
, args->args[i]
points to the argument
value. (See below for instructions on how to access the value properly.)
For a non-constant argument, args->args[i]
is 0
.
A constant argument is an expression that uses only constants, such as
3
or 4*7-2
or SIN(3.14)
. A non-constant argument is an
expression that refers to values that may change from row to row, such as
column names or functions that are called with non-constant arguments.
For each invocation of the main function, args->args
contains the
actual arguments that are passed for the row currently being processed.
Functions can refer to an argument i
as follows:
STRING_RESULT
is given as a string pointer plus a
length, to allow handling of binary data or data of arbitrary length. The
string contents are available as args->args[i]
and the string length
is args->lengths[i]
. You should not assume that strings are
null-terminated.
INT_RESULT
, you must cast
args->args[i]
to a long long
value:
long long int_val; int_val = *((long long*) args->args[i]);
REAL_RESULT
, you must cast
args->args[i]
to a double
value:
double real_val; real_val = *((double*) args->args[i]);
unsigned long *lengths
lengths
array indicates the
maximum string length for each argument. For each invocation of the main
function, lengths
contains the actual lengths of any string arguments
that are passed for the row currently being processed. For arguments of
types INT_RESULT
or REAL_RESULT
, lengths
still contains
the maximum length of the argument (as for the initialization function).
The initialization function should return 0
if no error occurred and
1
otherwise. If an error occurs, xxx_init()
should store a
null-terminated error message in the message
parameter. The message
will be returned to the client. The message buffer is
MYSQL_ERRMSG_SIZE
characters long, but you should try to keep the
message to less than 80 characters so that it fits the width of a standard
terminal screen.
The return value of the main function xxx()
is the function value, for
long long
and double
functions. For string functions, the
string is returned in the result
and length
arguments.
result
is a buffer at least 255 bytes long. Set these to the contents
and length of the return value. For example:
memcpy(result, "result string", 13); *length = 13;
The string function return value normally also points to the result.
To indicate a return value of NULL
in the main function, set
is_null
to 1
:
*is_null = 1;
To indicate an error return in the main function, set the error
parameter to 1
:
*error = 1;
If xxx()
sets *error
to 1
for any row, the function
value is NULL
for the current row and for any subsequent rows
processed by the statement in which XXX()
was invoked. (xxx()
will not even be called for subsequent rows.) Note: In
MySQL versions prior to 3.22.10, you should set both *error
and *is_null
:
*error = 1; *is_null = 1;
Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file `udf_example.cc' that is included in the MySQL source distribution. This file contains the following functions:
metaphon()
returns a metaphon string of the string argument.
This is something like a soundex string, but it's more tuned for English.
myfunc_double()
returns the sum of the ASCII values of the
characters in its arguments, divided by the sum of the length of its arguments.
myfunc_int()
returns the sum of the length of its arguments.
lookup()
returns the IP number for a hostname.
reverse_lookup()
returns the hostname for an IP number.
The function may be called with a string "xxx.xxx.xxx.xxx"
or
four numbers.
A dynamically-loadable file should be compiled as a sharable object file, using a command something like this:
shell> gcc -shared -o udf_example.so myfunc.cc
You can easily find out the correct compiler options for your system by running this command in the `sql' directory of your MySQL source tree:
shell> make udf_example.o
You should run a compile command similar to the one that make
displays,
except that you should remove the -c
option near the end of the line
and add -o udf_example.so
to the end of the line. (On some systems,
you may need to leave the -c
on the command.)
Once you compile a shared object containing UDFs, you must install it
and tell MySQL about it. Compiling a shared object from
`udf_example.cc' produces a file named something like
`udf_example.so' (the exact name may vary from platform to platform).
Copy this file to some directory searched by ld
, such as
`/usr/lib'. On many systems, you can set the LD_LIBRARY
or
LD_LIBRARY_PATH
environment variable to point at the directory where
you have your UDF function files. The dopen
manual page tells you
which variable you should use on your system. You should set this in
mysql.server
or safe_mysqld
and restart mysqld
.
After the library is installed, notify mysqld
about the new
functions with these commands:
mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "udf_example.so"; mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME "udf_example.so"; mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME "udf_example.so"; mysql> CREATE FUNCTION lookup RETURNS STRING SONAME "udf_example.so"; mysql> CREATE FUNCTION reverse_lookup RETURNS STRING SONAME "udf_example.so";
Functions can be deleted using DROP FUNCTION
:
mysql> DROP FUNCTION metaphon; mysql> DROP FUNCTION myfunc_double; mysql> DROP FUNCTION myfunc_int; mysql> DROP FUNCTION lookup; mysql> DROP FUNCTION reverse_lookup;
The CREATE FUNCTION
and DROP FUNCTION
statements update the
system table func
in the mysql
database. The function's name,
type and shared library name are saved in the table. You must have the
insert and delete privileges for the mysql
database
to create and drop functions.
You should not use CREATE FUNCTION
to add a function that has already
been created. If you need to reinstall a function, you should remove it with
DROP FUNCTION
and then reinstall it with CREATE FUNCTION
. You
would need to do this, for example, if you recompile a new version of your
function, so that mysqld
gets the new version. Otherwise the server
will continue to use the old version.
Active functions are reloaded each time the server starts, unless you start
mysqld
with the --skip-grant-tables
option. In this case, UDF
initialization is skipped and UDFs are unavailable. (An active function is
one that has been loaded with CREATE FUNCTION
and not removed with
DROP FUNCTION
.)
The procedure for adding a new native function is described below. Note that you cannot add native functions to a binary distribution since the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (e.g., when a new version is released), you will need to repeat the procedure with the new version.
To add a new native MySQL function, follow these steps:
sql_functions[]
array.
yacc
should define (this should be added at the
beginning of the file). Then define the function parameters and add an
``item'' with these parameters to the simple_expr
parsing rule.
For an example, check all occurrences of SOUNDEX
in
`sql_yacc.yy' to see how this is done.
Item_num_func
or
Item_str_func
, depending on whether your function returns a number or a
string.
double Item_func_newname::val() longlong Item_func_newname::val_int() String *Item_func_newname::Str(String *str)
void Item_func_newname::fix_length_and_dec()This function should at least calculate
max_length
based on the
given arguments. max_length
is the maximum number of characters
the function may return. This function should also set maybe_null = 0
if the main function can't return a NULL
value. The function can check
if any of the function arguments can return NULL
by checking the
arguments maybe_null
variable.
All functions must be thread-safe.
For string functions, there are some additional considerations to be aware of:
String *str
argument provides a string
buffer that may be used to hold the result.
In MySQL, you can define a procedure in C++ that can access and
modify the data in a query before it is sent to the client. The modification
can be done on row by row or GROUP BY
level.
We have created an example procedure in MySQL 3.23 to show you what can be done.
analyse([max elements,[max memory]])
This procedure is defined in the `sql/sql_analyse.cc'. This examines the result from your query and returns an analysis of the results.
max elements
(default 256) is the maximum number of distinct values
analyse
will notice per column. This is used by analyse
to check if
the optimal column type should be of type ENUM
.
max memory
(default 8192) is the maximum memory analyse
should
allocate per column while trying to find all distinct values.
SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])
For the moment, the only documentation for this is the source. :(
You can find all information about procedures by examining the following files:
MySQL provides support for ODBC by means of the MyODBC program.
MyODBC is a 32-bit ODBC (2.50) level 0 driver for Windows95 and Windows NT. We hope somebody will port it to Windows 3.x.
MyODBC has been tested with Access, Admndemo.exe, C++-Builder, Centura Team Developer (formerly Gupta SQL/Windows), ColdFusion (on Solaris and NT with svc pack 5), Crystal Reports, DataJunction, Delphi, ERwin, Excel, iHTML, FileMaker Pro, FoxPro, Notes 4.5/4.6, SBSS, Perl DBD-ODBC, Paradox, Powerbuilder, Powerdesigner 32 bit, VC++ and Visual Basic.
If you know of any other applications that work with MyODBC, please mail myodbc@lists.mysql.com about this!
Most programs should work with MyODBC, but for each of those listed below, we have tested it ourselves or gotten confirmation from some user that it works:
BLOB
columns as OLE OBJECTS
. If
you want to have MEMO
columns instead, you should change the
column to TEXT
with ALTER TABLE
.
DATE
columns properly. If you have a problem
with these, change the columns to DATETIME
.
"Query|SQLSpecific|Pass-Through"
from the Access menu.
VARCHAR
rather than ENUM
, as
it exports the latter in a manner that causes MySQL grief.
CONCAT()
function. For example:
select CONCAT(rise_time), CONCAT(set_time) from sunrise_sunset;Values retrieved as strings this way should be correctly recognized as time values by Excel97. The purpose of
CONCAT()
in this example is to fool ODBC into thinking
the column is of ``string type''. Without the CONCAT()
, ODBC knows the
column is of time type, and Excel does not understand that.
Note that this is a bug in Excel, because it automatically converts a
string to a time. This would be great if the source was a text file, but
is plain stupid when the source is an ODBC connection that reports
exact types for each column.
fReg:= TRegistry.Create; fReg.OpenKey('\Software\ODBC\ODBC.INI\DocumentsFab', True); fReg.WriteString('Database', 'Documents'); fReg.WriteString('Description', ' '); fReg.WriteString('Driver', 'C:\WINNT\System32\myodbc.dll'); fReg.WriteString('Flag', '1'); fReg.WriteString('Password', ''); fReg.WriteString('Port', ' '); fReg.WriteString('Server', 'xmark'); fReg.WriteString('User', 'winuser'); fReg.OpenKey('\Software\ODBC\ODBC.INI\ODBC Data Sources', True); fReg.WriteString('DocumentsFab', 'MySQL'); fReg.CloseKey; fReg.Free; Memo1.Lines.Add('DATABASE NAME='); Memo1.Lines.Add('USER NAME='); Memo1.Lines.Add('ODBC DSN=DocumentsFab'); Memo1.Lines.Add('OPEN MODE=READ/WRITE'); Memo1.Lines.Add('BATCH COUNT=200'); Memo1.Lines.Add('LANGDRIVER='); Memo1.Lines.Add('MAX ROWS=-1'); Memo1.Lines.Add('SCHEMA CACHE DIR='); Memo1.Lines.Add('SCHEMA CACHE SIZE=8'); Memo1.Lines.Add('SCHEMA CACHE TIME=-1'); Memo1.Lines.Add('SQLPASSTHRU MODE=SHARED AUTOCOMMIT'); Memo1.Lines.Add('SQLQRYMODE='); Memo1.Lines.Add('ENABLE SCHEMA CACHE=FALSE'); Memo1.Lines.Add('ENABLE BCD=FALSE'); Memo1.Lines.Add('ROWSET SIZE=20'); Memo1.Lines.Add('BLOBS TO CACHE=64'); Memo1.Lines.Add('BLOB SIZE=32'); AliasEditor.Add('DocumentsFab','MySQL',Memo1.Lines);
There are three possibilities for specifying the server name on Windows95:
ip hostnameFor example:
194.216.84.21 my
Example of how to fill in the ``ODBC setup'':
Windows DSN name: test Description: This is my test database MySql Database: test Server: 194.216.84.21 User: monty Password: my_password Port:
The value for the Windows DSN name
field is any name that is unique
in your Windows ODBC setup.
You don't have to specify values for the Server
, User
,
Password
or Port
fields in the ODBC setup screen.
However, if you do, the values will be used as the defaults later when
you attempt to make a connection. You have the option of changing the
values at that time.
If the port number is not given, the default port (3306) is used.
If you specify the option Read options from C:\my.cnf
, the groups
client
and odbc
will be read from the `C:\my.cnf' file.
You can use all options that are usable by mysql_options()
.
See section 20.4.37 mysql_options()
.
AUTO_INCREMENT
column in ODBC
A common problem is how to get the value of an automatically generated ID
from an INSERT
. With ODBC, you can do something like this (assuming
that auto
is an AUTO_INCREMENT
field):
INSERT INTO foo (auto,text) VALUES(NULL,'text'); SELECT LAST_INSERT_ID();
Or, if you are just going to insert the ID into another table, you can do this:
INSERT INTO foo (auto,text) VALUES(NULL,'text'); INSERT INTO foo2 (id,text) VALUES(LAST_INSERT_ID(),'text');
For the benefit of some ODBC applications (at least Delphi and Access), the following query can be used to find a newly-inserted row:
SELECT * FROM tbl_name WHERE auto IS NULL;
If you encounter difficulties with MyODBC, you should start by making a
log file from the ODBC manager (the log you get when requesting logs
from ODBCADMIN) and a MyODBC log. To get a MyODBC log,
tag the `Trace MyODBC' option flag in the MyODBC
connect/configure screen. The log will be written to file
`C:\myodbc.log'. Note that you must use MYSQL.DLL
and not
MYSQL2.DLL
for this option to work!
Check the queries that MyODBC sends to the MySQL server; You
should be able to find this by searching after the string
>mysql_real_query
in the `myodbc.log' file.
You should also try duplicating the queries in the mysql
monitor
or admndemo
to find out if the error is MyODBC or MySQL.
If you find out something is wrong, please only send the relevant rows (max 40 rows) to the myodbc@lists.mysql.com. Please never send the whole MyODBC or ODBC log file !
If you are unable to find out what's wrong, the last option is to to make a archive (tar or zip) that contains a MyODBC log file, the ODBC log file and a README file that explains the problem. You can send this to ftp://www.mysql.com/pub/mysql/secret. Only we at TCX will have access to the files you upload and we will be very discrete with the data!
If you can create a program that also shows this problem, please upload this too!
If the program works with some other SQL server, you should make a ODBC log file where you do exactly the same thing in the other SQL server.
Remember that the more information you can supply to us, the more likely it is that we can fix the problem!
MySQL
with some common programsThe contrib section includes programs that lets you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. See section B Contributed programs.
You can change the Apache logging format to be easily readable by
MySQL
by putting the following into the Apache configuration file:
LogFormat \ "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\", \ \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""
In MySQL you can now do something like this:
LOAD DATA INFILE '/local/access_log' INTO TABLE table_name FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'
All MySQL versions are tested on many platforms before they are released. This doesn't mean that there isn't any bugs in MySQL, but that if there are bugs they are very few and can be hard to find. If you have a problem, it will always help if you try to find out exactly what crashes your system as you will have a much better chance of getting this fixed quickly.
First you should try to find out whether the problem is that the
mysqld
daemon dies or whether your problem has to do with your
client. You can check how long your mysqld
server has been up by
executing mysqladmin version
. If mysqld
has died, you may
find the reason for this in the file
`mysql-data-directory/'hostname'.err'.
Since it is very difficult to know why something is crashing, first try to check whether or not things that work for others crash for you. Please try the following things:
mysqld
daemon with mysqladmin shutdown
, run
myisamchk --silent --force */*.MYI
on all tables and restart the
mysqld
daemon. This will ensure that you are running from a clean
state. See section 13 Maintaining a MySQL installation.
mysqld --log
and try to determine from the information in the log
whether or not some specific query kills the server. 95% of all bugs are
related to a particular query! Normally this is one of the last queries in
the log file just before MySQL restarted.
You may be able to verify this using the following procedure:
mysqladmin shutdown
)
myisamchk -s */*.MYI
to verify that all tables
are correct. If any table is corrupted, repair it with
myisamchk -r path-to-table.MYI
.
safe_mysql --log
.
mysqld
now dies, you can test if the problem is a specific
query by restoring the backup and executing mysql <
mysql-log-file
. You can of course do the last test in some other
directory than the standard MySQL database directory by
starting another MySQL server with safe_mysqld
--data=path-to-backup-directory
.
fork_test.pl
and fork2_test.pl
.
--with-debug
option to
configure
and then recompile. See section G.1 Debugging a MySQL server.
--skip-locking
option to mysqld
. On some systems, the
lockd
lock manager does not work properly; the --skip-locking
option tells mysqld
not to use external locking. (This means that you
cannot run 2 mysqld
servers on the same data and that you must be
careful if you use myisamchk
, but it may be instructive to try the
option as a test.)
mysqladmin -u root processlist
when mysqld
appears to be running but not responding? Sometimes mysqld
is not
comatose even though you might think so. The problem may be that all
connections are in use, or there may be some internal lock problem.
mysqladmin processlist
will usually be able to make a connection even
in these cases, and can provide useful information about the current number
of connections and their status.
mysqladmin -i 5 status
in a separate window to produce statistics while you run your other queries.
mysqld
from gdb
(or another debugger).
back
(or the backtrace command in your debugger) when
mysqld
core dumps.
BLOB/TEXT
columns (but only VARCHAR
columns) you
can try to change all VARCHAR
to CHAR
with ALTER
TABLE
. This will force MySQL to use fixed size rows. Fixed
size rows take a little extra place, but are much more tolerant for
corruption!
The current dynamic row code has been in use at TCX for at least 3
years without any problems, but by nature dynamic length rows are more
prone to errors so it may be a good idea to try if the above helps!
MySQL server has gone away
error
This section also covers the related Lost connection to server
during query
error.
The most common reason for the MySQL server has gone away
error
is that the server timed out and closed the connection. By default, the
server closes the connection after 8 hours if nothing has happened. You
can change the time limit with by setting the wait_timeout
variable when
you start mysqld.
You can check that the MySQL hasn't died by executing
mysqladmin version
and examining the uptime.
If you have a script, you just have to issue the query again for the client to do an automatic reconnection.
You normally can get the following error codes in this case (which one you get is OS-dependent):
CR_SERVER_GONE_ERROR | The client couldn't send a question to the server. |
CR_SERVER_LOST | The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question. |
You can also get these errors if you send a query to the server that is
incorrect or too large. If mysqld
gets a packet that is too large
or out of order, it assumes that something has gone wrong with the client and
closes the connection. If you need big queries (for example, if you are
working with big BLOB
columns), you can increase the query limit by
starting mysqld
with the -O max_allowed_packet=#
option
(default 1M). The extra memory is allocated on demand, so mysqld
will
use more memory only when you issue a big query or when mysqld
must
return a big result row!
Can't connect to [local] MySQL server
error
A MySQL client can connect to the mysqld
server in two
different ways: Unix sockets, which connect through a file in the file
system (default `/tmp/mysqld.sock'), or TCP/IP, which connects
through a port number. Unix sockets are faster than TCP/IP but can only
be used when connecting to a server on the same computer. Unix sockets
are used if you don't specify a hostname or if you specify the special
hostname localhost
.
The error (2002) Can't connect to ...
normally means that there
isn't a MySQL server running on the system or that you are
using a wrong socket file or TCP/IP port when trying to connect to the
mysqld
server.
Start by check (using ps
) that there is a process running named
mysqld
on your server! If there isn't any mysqld
process, you
should start one. See section 4.15.2 Problems starting the MySQL server.
If a mysqld
process is running, you can check the server by
trying these different connections (the port number and socket pathname
might be different in your setup, of course):
shell> mysqladmin version shell> mysqladmin variables shell> mysqladmin -h `hostname` version variables shell> mysqladmin -h `hostname` --port=3306 version shell> mysqladmin -h 'ip for your host' version shell> mysqladmin --socket=/tmp/mysql.sock version
Note the use of backquotes rather than forward quotes with the hostname
command; these cause the output of hostname
(i.e., the current
hostname) to be substituted into the mysqladmin
command.
Here are some reasons the Can't connect to local MySQL server
error might occur:
mysqld
is not running.
mysqld
uses the MIT-pthreads package.
See section 4.2 Operating systems supported by MySQL.
However, MIT-pthreads doesn't support Unix sockets, so on such a system you
must always specify the hostname explicitly when connecting to the server.
Try using this command to check the connection to the server:
shell> mysqladmin -h `hostname` version
mysqld
uses (default
`/tmp/mysqld.sock'). You might have a cron
job that removes the
MySQL socket (e.g., a job that removes old files from the `/tmp'
directory). You can always run mysqladmin version
and
check that the socket mysqladmin
is trying to use really exists.
The fix in this case is to change the cron
job to not remove
`mysqld.sock' or to place the socket somewhere else. You can specify
a different socket location at MySQL configuration time with this
command:
shell> ./configure --with-unix-socket-path=/path/to/socketYou can also start
safe_mysqld
with the
--socket=/path/to/socket
option and set the environment variable
MYSQL_UNIX_PORT
to the socket pathname before starting your
MySQL clients. @item You have started the mysqld
server with
the --socket=/path/to/socket
option. If you change the socket
pathname for the server, you must also notify the MySQL clients
about the new path. You can do this by setting the environment variable
MYSQL_UNIX_PORT
to the socket pathname or by providing the socket path
as an argument to the clients. You can test the socket with this command:
shell> mysqladmin --socket=/path/to/socket version
mysqld
threads (for example with the
mysql_zap
script before you can start a new MySQL
server. See section 18.1 What to do if MySQL keeps crashing.
If you get the error message Can't connect to MySQL server on
some_hostname
, you can try the following things to find out what is the
problem:
telnet your-host-name
tcp-ip-port-number
and press RETURN
a couple of times. If there
is a MySQL server running on this port you should get a
responses that includes the version number of the running MySQL
server. If you get an error like telnet: Unable to connect to
remote host: Connection refused
, then there is no server running on the
used port.
mysqld
daemon on the local machine and check
the TCP/IP port that mysqld it's configured to use (variable port
) with
mysqladmin variables
.
mysqld
server is not started with the
--skip-networking
option.
Host '...' is blocked
errorIf you get a error like this:
Host 'hostname' is blocked because of many connection errors. Unblock with 'mysqladmin flush-hosts'
This means that mysqld
has gotten a lot (max_connect_errors
)
of connect requests from the host 'hostname'
that have been interrupted
in the middle. After max_connect_errors
failed requests, mysqld
assumes that something is wrong (like a attack from a cracker), and
blocks the site from further connections until someone executes the command
mysqladmin flush-hosts
.
By default, mysqld
blocks a host after 10 connection errors.
You can easily adjust this by starting the server like this:
shell> safe_mysqld -O max_connect_errors=10000 &
Note that if you get this error message for a given host, you should first
check that there isn't anything wrong with TCP/IP connections from that
host. If your TCP/IP connections aren't working, it won't do you any good to
increase the value of the max_connect_errors
variable!
Out of memory
errorIf you issue a query and get something like the following error:
mysql: Out of memory at line 42, 'malloc.c' mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k) ERROR 2008: MySQL client ran out of memory
Note that the error refers to the MySQL client mysql
. The
reason for this error is simply that the client does not have enough memory to
store the whole result.
To remedy the problem, first check that your query is correct. Is it
reasonable that it should return so many rows? If so,
you can use mysql --quick
, which uses mysql_use_result()
to retrieve the result set. This places less of a load on the client (but
more on the server).
Packet too large
error
When a MySQL client or the mysqld
server gets a packet bigger
than max_allowed_packet
bytes, it issues a Packet too large
error and closes the connection.
If you are using the mysql
client, you may specify a bigger buffer by
starting the client with mysql --set-variable=max_allowed_packet=8M
.
If you are using other clients that do not allow you to specify the maximum
packet size (such as DBI
), you need to set the packet size when you
start the server. You cau use a command-line option to mysqld
to set
max_allowed_packet
to a larger size. For example, if you are
expecting to store the full length of a BLOB
into a table, you'll need
to start the server with the --set-variable=max_allowed_packet=24M
option.
The table is full
error
This error occurs when an in-memory temporary table becomes larger than
tmp_table_size
bytes. To avoid this problem, you can use the
-O tmp_table_size=#
option to mysqld
to increase the
temporary table size, or use the SQL option SQL_BIG_TABLES
before
you issue the problematic query. See section 7.25 SET OPTION
syntax.
You can also start mysqld
with the --big-tables
option.
This is exactly the same as using SQL_BIG_TABLES
for all queries.
Commands out of sync
error in client
If you get Commands out of sync; You can't run this command now
in your client code, you are calling client functions in the wrong order!
This can happen, for example, if you are using mysql_use_result()
and
try to execute a new query before you have called mysql_free_result()
.
It can also happen if you try to execute two queries that return data without
a mysql_use_result()
or mysql_store_result()
in between.
Ignoring user
errorIf you get the following error:
Found wrong password for user: 'some_user@some_host'; Ignoring user
This means that when mysqld
was started or when it reloaded the
permissions tables, it found an entry in the user
table with
an invalid password. As a result, the entry is simply ignored by the
permission system.
Possible causes of and fixes for this problem:
mysqld
with an old
user
table.
You can check this by executing mysqlshow mysql user
to see if
the password field is shorter than 16 characters. If so, you can correct this
condition by running the scripts/add_long_password
script.
mysqld
with the --old-protocol
option.
Update the user in the user
table with a new password or
restart mysqld
with --old-protocol
.
user
table without using the
PASSWORD()
function. Use mysql
to update the user in the
user
table with a new password. Make sure to use the PASSWORD()
function:
mysql> update user set password=PASSWORD('your password') where user='XXX';
Table 'xxx' doesn't exist
error
If you get the error Table 'xxx' doesn't exist
or Can't
find file: 'xxx' (errno: 2)
, this means that no table exists
in the current database with the name xxx
.
Note that as MySQL uses directories and files to store databases and tables, the database and table names are case sensitive! (On Win32 the databases and tables names are not case sensitive, but all references to a given table within a query must use the same case!)
You can check which tables you have in the current database with
SHOW TABLES
. See section 7.21 SHOW
syntax (Get information about tables, columns,...).
When a disk full condition occurs, MySQL does the following:
To alleviate the problem, you can take the following actions:
mysqladmin kill
to the thread.
The thread will be aborted the next time it checks the disk (in 1 minute).
The mysql
client typically is used interactively, like this:
shell> mysql database
However, it's also possible to put your SQL commands in a file and tell
mysql
to read its input from that file. To do so, create a text
file `text_file' that contains the commands you wish to execute.
Then invoke mysql
as shown below:
shell> mysql database < text_file
You can also start your text file with a USE db_name
statement. In
this case, it is unnecessary to specify the database name on the command
line:
shell> mysql < text_file
See section 12.1 Overview of the different MySQL programs.
MySQL uses the value of the TMPDIR
environment variable as
the pathname of the directory in which to store temporary files. If you don't
have TMPDIR
set, MySQL uses the system default, which is
normally `/tmp' or `/usr/tmp'. If the file system containing your
temporary file directory is too small, you should edit safe_mysqld
to
set TMPDIR
to point to a directory in a file system where you have
enough space! You can also set the temporary directory using the
--tmpdir
option to mysqld
.
MySQL creates all temporary files as ``hidden files''. This ensures
that the temporary files will be removed if mysqld
is terminated. The
disadvantage of using hidden files is that you will not see a big temporary
file that fills up the file system in which the temporary file directory is
located.
When sorting (ORDER BY
or GROUP BY
), MySQL normally
uses one or two temporary files. The maximum disk-space needed is:
(length of what is sorted + sizeof(database pointer)) * number of matched rows * 2
sizeof(database pointer)
is usually 4, but may grow in the future for
really big tables.
For some SELECT
queries, MySQL also creates temporary SQL
tables. These are not hidden and have names of the form `SQL_*'.
ALTER TABLE
and OPTIMIZE TABLE
create a temporary table in the
same directory as the original table.
If you have problems with the fact that anyone can delete the
MySQL communication socket `/tmp/mysql.sock', you can,
on most versions of Unix, protect your `/tmp' file system by setting
the sticky
bit on it. Log in as root
and do the following:
shell> chmod +t /tmp
This will protect your `/tmp' file system so that files can be deleted
only by their owners or the superuser (root
).
You can check if the sticky
bit is set by executing ls -ld /tmp
.
If the last permission bit is t
, the bit is set.
Access denied
error
See section 6.6 How the privilege system works. And especially see section 6.13 Causes of Access denied
errors.
The MySQL server mysqld
can be started and run by any user.
In order to change mysqld
to run as Unix user user_name
, you must
do the following:
mysqladmin shutdown
).
user_name
has
privileges to read and write files in them (you may need to do this as
the Unix root
user):
shell> chown -R user_name /path/to/mysql/datadirIf directories or files within the MySQL data directory are symlinks, you'll also need to follow those links and change the directories and files they point to.
chown -R
may not follow symlinks for
you.
user_name
, or, if you are using
MySQL 3.22 or later, start mysqld
as the Unix root
user and use the --user=user_name
option. mysqld
will switch
to run as Unix user user_name
before accepting any connections.
mysql.server
script to start mysqld
when
the system is rebooted, you should edit mysql.server
to use su
to run mysqld
as user user_name
, or to invoke mysqld
with the --user
option. (No changes to safe_mysqld
are
necessary.)
At this point, your mysqld
process should be running fine and dandy as
the Unix user user_name
. One thing hasn't changed, though: the
contents of the permissions tables. By default (right after running the
permissions table install script mysql_install_db
), the MySQL
user root
is the only user with permission to access the mysql
database or to create or drop databases. Unless you have changed those
permissions, they still hold. This shouldn't stop you from accessing
MySQL as the MySQL root
user when you're logged in
as a Unix user other than root
; just specify the -u root
option
to the client program.
Note that accessing MySQL as root
, by supplying -u
root
on the command line, has nothing to do with MySQL running
as the Unix root
user, or, indeed, as other Unix user. The access
permissions and user names of MySQL are completely separate from
Unix user names. The only connection with Unix user names is that if you
don't provide a -u
option when you invoke a client program, the client
will try to connect using your Unix login name as your MySQL user
name.
If your Unix box itself isn't secured, you should probably at least put a
password on the MySQL root
users in the access tables.
Otherwise, any user with an account on that machine can run mysql -u
root db_name
and do whatever he likes.
If you have forgotten the root
user password for MySQL, you
can restore it with the following procedure.
kill
(not kill
-9
) to the mysqld
server. The pid is stored in a .pid
file which is normally in the MySQL database directory:
kill `cat /mysql-data-directory/hostname.pid`You must be either the UNIX
root
user or the same user the server
runs as to do this.
mysqld
with the --skip-grant-tables
option.
mysql -h hostname mysql
and change
the password with a GRANT
command. See section 7.26 GRANT
and REVOKE
syntax.
You can also do this with
mysqladmin -h hostname -u user password 'new password'
mysqladmin -h hostname
flush-privileges
or with the SQL command FLUSH PRIVILEGES
.
If you have problems with file permissions, for example, if mysql
issues the following error message when you create a table:
ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)
Then the environment variable UMASK
might be set incorrectly when
mysqld
starts up. The default umask value is 0660
. You can
change this behavior by starting safe_mysqld
as follows:
shell> UMASK=384 # = 600 in octal shell> export UMASK shell> /path/to/safe_mysqld &
If you get ERROR '...' not found (errno: 23)
, Can't open
file: ... (errno: 24)
or any other error with errno 23
or
errno 24
from MySQL, it means that you haven't allocated
enough file descriptors for MySQL. You can use the
perror
utility to get a description of what the error number
means:
shell> perror 23 File table overflow shell> perror 24 Too many open files
The problem here is that mysqld
is trying to keep open too many
files simultaneously. You can either tell mysqld
not to open so
many files at once, or increase the number of file descriptors
available to mysqld
.
To tell mysqld
to keep open fewer files at a time, you can make the
table cache smaller by using the -O table_cache=32
option
to safe_mysqld
(the default value is 64). Reducing the value of
max_connections
will also reduce the number of open files (the default
value is 90).
To change the number of file descriptors available to mysqld
, modify
the safe_mysqld
script. There is a commented-out line
ulimit -n 256
in the script. You can remove the '#'
character
to uncomment this line, and change the number 256 to change the number of
file descriptors available to mysqld
.
ulimit
can increase the number of file descriptors, but only up to the
limit imposed by the operating system. If you need to increase the OS limit
on the number of file descriptors available to each process, consult the
documentation for your operating system.
Note that if you run the tcsh
shell, ulimit
will not work!
tcsh
will also report incorrect values when you ask for the current
limits! In this case you should start safe_mysqld
with sh
!
DATE
columns
The format of a DATE
value is 'YYYY-MM-DD'
. According to ANSI
SQL, no other format is allowed. You should use this format in UPDATE
expressions and in the WHERE clause of SELECT
statements. For
example:
mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05';
As a convenience, MySQL automatically converts a date to a number if
the date is used in a numeric context (and vice versa). It is also smart
enough to allow a ``relaxed'' string form when updating and in a WHERE
clause that compares a date to a TIMESTAMP
, DATE
or a
DATETIME
column. (Relaxed form means that any punctuation character
may be used as the separator between parts. For example, '1998-08-15'
and '1998#08#15'
are equivalent.) MySQL can also convert a
string containing no separators (such as '19980815'
), provided it
makes sense as a date.
The special date '0000-00-00'
can be stored and retrieved as
'0000-00-00'.
When using a '0000-00-00'
date through
MyODBC, it will automatically be converted to NULL
in
MyODBC 2.50.12 and above, because ODBC can't handle this kind of
date.
Since MySQL performs the conversions described above, the following statements work:
mysql> INSERT INTO tbl_name (idate) VALUES (19970505); mysql> INSERT INTO tbl_name (idate) VALUES ('19970505'); mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05'); mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05'); mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05'); mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00'); mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05'; mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505; mysql> SELECT mod(idate,100) FROM tbl_name WHERE idate >= 19970505; mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';
However, the following will not work:
mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0;
STRCMP()
is a string function, so it converts idate
to
a string and performs a string comparison. It does not convert
'19970505'
to a date and perform a date comparison.
Note that MySQL does no checking whether or not the date is
correct. If you store an incorrect date, such as '1998-2-31'
, the
wrong date will be stored. If the date cannot be converted to any reasonable
value, a 0
is stored in the DATE
field. This is mainly a speed
issue and we think it is up to the application to check the dates, and not
the server.
If you have a problem with SELECT NOW()
returning values in GMT and
not your local time, you have to set the TZ
environment variable to
your current timezone. This should be done for the environment in which
the server runs, for example in safe_mysqld
or mysql.server
.
By default, MySQL searches are case-insensitive (although there are
some character sets that are never case insensitive, such as czech
).
That means that if you search with col_name LIKE 'a%'
, you will get all
column values that start with A
or a
. If you want to make this
search case-sensitive, use something like INDEX(col_name, "A")=0
to
check a prefix. Or use STRCMP(col_name, "A") = 0
if the column value
must be exactly "A"
.
Simple comparison operations (>=, >, = , < , <=
, sorting and
grouping) are based on each character's ``sort value''. Characters with
the same sort value (like E, e and 'e) are treated as the same character!
LIKE
comparisons are done on the uppercase value of each character
(E == e but E <> 'e)
If you want a column always to be treated in case-sensitive fashion,
declare it as BINARY
. See section 7.7 CREATE TABLE
syntax.
If you are using Chinese data in the so-called big5 encoding, you want to
make all character columns BINARY
. This works because the sorting
order of big5 encoding characters is based on the order of ASCII codes.
NULL
values
The concept of the NULL
value is a common source of confusion for
newcomers to SQL, who often think that NULL
is the same thing as an
empty string ''
. This is not the case! For example, the following
statements are completely different:
mysql> INSERT INTO my_table (phone) VALUES (NULL); mysql> INSERT INTO my_table (phone) VALUES ("");
Both statements insert a value into the phone
column, but the first
inserts a NULL
value and the second inserts an empty string. The
meaning of the first can be regarded as ``phone number is not known'' and the
meaning of the second can be regarded as ``she has no phone''.
In SQL, the NULL
value is always false in comparison to any
other value, even NULL
. An expression that contains NULL
always produces a NULL
value unless otherwise indicated in
the documentation for the operators and functions involved in the
expression. All columns in the following example return NULL
:
mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);
If you want to search for column values that are NULL
, you
cannot use the =NULL
test. The following statement returns no
rows, because expr = NULL
is FALSE, for any expression:
mysql> SELECT * FROM my_table WHERE phone = NULL;
To look for NULL
values, you must use the IS NULL
test.
The following shows how to find the NULL
phone number and the
empty phone number:
mysql> SELECT * FROM my_table WHERE phone IS NULL; mysql> SELECT * FROM my_table WHERE phone = "";
In MySQL, as in many other SQL servers, you can't index
columns that can have NULL
values. You must declare such columns
NOT NULL
. Conversely, you cannot insert NULL
into an indexed
column.
When reading data with LOAD DATA INFILE
, empty columns are updated
with ''
. If you want a NULL
value in a column, you should use
\N
in the text file. The literal word 'NULL'
may also be used
under some circumstances.
See section 7.16 LOAD DATA INFILE
syntax.
When using ORDER BY
, NULL
values are presented first. If you
sort in descending order using DESC
, NULL
values are presented
last. When using GROUP BY
, all NULL
values are regarded as
equal.
To help with NULL
handling, you can use the IS NULL
and
IS NOT NULL
operators and the IFNULL()
function.
For some column types, NULL
values are handled specially. If you
insert NULL
into the first TIMESTAMP
column of a table, the
current date and time is inserted. If you insert NULL
into an
AUTO_INCREMENT
column, the next number in the sequence is inserted.
alias
You can use alias to refer to a column in the GROUP BY
,
ORDER BY
or in the HAVING
part. Aliases can also be used
to give columns more better names:
SELECT SQRT(a*b) as rt FROM table_name GROUP BY rt HAVING rt > 0; SELECT id,COUNT(*) AS cnt FROM table_name GROUP BY id HAVING cnt > 0; SELECT id AS "Customer identity" FROM table_name;
Note that you ANSI SQL doesn't allow you to refer to an alias in a
WHERE
clause. This is because that when the WHERE
code is
executed the column value may not yet be determinated. For example the
following query is illegal:
SELECT id,COUNT(*) AS cnt FROM table_name WHERE cnt > 0 GROUP BY id;
The WHERE
statement is executed to determinate which rows should
be included in the GROUP BY
part while HAVING
is used to
decide which rows from the result set should be used.
As MySQL doesn't support sub-selects or use of more than one table
in the DELETE
statement, you should use the following approach to
delete rows from 2 related tables:
SELECT
the rows based on some WHERE
condition in the main table.
DELETE
the rows in the main table based on the same condition.
DELETE FROM related_table WHERE related_column IN (selected_rows)
If the total number of characters in the query with
related_column
is more than 1,048,576 (the default value of
max_allowed_packet
, you should split it into smaller parts and
execute multiple DELETE
statements. You will probably get the
fastest DELETE
by only deleting 100-1000 related_column
id's per time if the related_column
is an index. If the
related_column
isn't an index, the speed is independent of the
number of arguments in the IN
clause.
If you have a complicated query with many tables that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:
EXPLAIN
and check if you can find something that is
obviously wrong. See section 7.22 EXPLAIN
syntax (Get information about a SELECT
).
WHERE
clause.
LIMIT 10
with the query.
SELECT
for the column that should have matched a row,
against the table that was last removed from the query.
FLOAT
or DOUBLE
columns with numbers that
have decimals, you can't use =
! This problem is common in most
computer languages because floating point values are not exact values.
mysql> SELECT * FROM table_name WHERE float_column=3.5; -> mysql> SELECT * FROM table_name WHERE float_column between 3.45 and 3.55;In most cases, changing the
FLOAT
to a DOUBLE
will fix this!
mysql test < query.sql
that shows your problems.
You can create a test file with mysqldump --quick database tables > query.sql
. Take the file up in a editor, remove some insert lines (if there are
too many of these) and add your select statement last in the file.
Test that you still have your problem by doing:
shell> mysqladmin create test2 shell> mysql test2 < query.sqlPost the test file using
mysqlbug
to mysql@lists.mysql.com.
ALTER TABLE
.
If ALTER TABLE
dies with an error like this:
Error on rename of './database/name.frm' to './database/B-a.frm' (Errcode: 17)
The problem may be that MySQL has crashed in a previous ALTER
TABLE
and there is an old table named `A-something' or
`B-something' lying around. In this case, go to the MySQL data
directory and delete all files that have names starting with A-
or
B-
. (You may want to move them elsewhere instead of deleting them).
ALTER TABLE
works the following way:
If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (this shouldn't happen, of course), MySQL may leave the old table as `B-xxx' but a simple rename should get your data back.
The whole point of SQL is to abstract the application from the data storage format. You should always specify the order in wish you wish to retrieve your data. For example:
SELECT col_name1, col_name2, col_name3 FROM tbl_name;
will return columns in the order col_name1
, col_name2
, col_name3
, whereas:
SELECT col_name1, col_name3, col_name2 FROM tbl_name;
will return columns in the order col_name1
, col_name3
, col_name2
.
You should NEVER, in an application, use SELECT *
and
retrieve the columns based on their position, because the order in which columns are
returned CANNOT be guaranteed over time; A simple change to
your database may cause your application to fail rather dramatically.
If you want to change the order of columns anyway, you can do it as follows:
INSERT INTO new_table SELECT fields-in-new_table-order FROM old_table
.
old_table
ALTER TABLE new_table RENAME old_table
One way replication can be used both to increase robustness and speed. For robustness you have two systems and switch to the backup if you get problems you witch to the backup. The extra speed is achieved by sending a part of the non updating queries to the replica server. Of course this only works if non updating queries dominate, but that is the normal case.
One way replication is planned for the near future. This will be implementat so that slave servers will be synced with low priority updates and delayed inserts up to date (this will give readers higher priority than writers).
MySQL doesn't (yet) have database replication, but here are some info on how do to it.
The most general way to replicate a database is to use the update
log. See section 9.2 The update log. This requires one database that acts as a master
(to which data changes are made) and one or more other databases that act
as slaves. To update a slave, just run mysql < update_log
.
Supply host, user and password options that are appropriate for the slave
database, and use the update log from the master database as input.
If you never delete anything from a table, you can use a TIMESTAMP
column to find out which rows have been inserted or changed in the table
since the last replication (by comparing to the time when you did the
replication last time) and only copy these rows to the mirror.
It is possible to make a two-way updating system using both the update log (for deletes) and timestamps (on both sides). But in that case you must be able to handle conflicts when the same data have been changed in both ends. You probably want to keep the old version to help with deciding what has been updated.
Because replication in this case is done with SQL statements, you should not use the following functions in statements that update the database; they may not return the same value as in the original database:
DATABASE()
GET_LOCK()
and RELEASE_LOCK()
RAND()
USER()
, SYSTEM_USER()
or SESSION_USER()
VERSION()
All time functions are safe to use, as the timestamp is sent to the
mirror if needed. LAST_INSERT_ID()
is also safe to use.
Since MySQL tables are stored as files, it is easy to do a
backup. To get a consistent backup, do a LOCK TABLES
on the
relevant tables. See section 7.24 LOCK TABLES/UNLOCK TABLES
syntax. You only need a
read lock; this allows other threads to continue to query the tables while
you are making a copy of the files in the database directory. If you want to
make a SQL level backup, you can use SELECT INTO OUTFILE
.
Another way to backup a database is to use the mysqldump
program:
shell> mysqldump --tab=/path/to/some/dir --opt --fullYou can also simply copy all table files (`*.frm', `*.MYD' and `*.MYI' files), as long as the server isn't updating anything.
mysqld
if it's running, then start it with the
--log-update
option. You will get log files with names of the
form `hostname.n', where n
is a number that is
incremented each time you execute mysqladmin refresh
or
mysqladmin flush-logs
, the FLUSH LOGS
statement, or
restart the server. These log files provide you with the information
you need to replicate changes to the database that are made subsequent
to the point at which you executed mysqldump
.
If you have to restore something, try to recover your tables using
myisamchk -r
first. That should work in 99.9% of all cases. If
myisamchk
fails, try the following procedure:
mysqldump
backup.
shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
ls
is used to get all the log files in the right order.
You can also do selective backups with SELECT * INTO OUTFILE 'file_name'
FROM tbl_name
and restore with LOAD DATA INFILE 'file_name' REPLACE
...
To avoid duplicate records, you need a PRIMARY KEY
or a
UNIQUE
key in the table. The REPLACE
keyword causes old records
to be replaced with new ones when a new record duplicates an old record on
a unique key value.
There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.
If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file.
Assume an existing server is configured for the default port number and
socket file. Then configure the new server with a configure
command
something like this:
shell> ./configure --with-tcp-port=port_number \ --with-unix-socket=file_name \ --prefix=/usr/local/mysql-3.22.9
Here port_number
and file_name
should be different than the
default port number and socket file pathname, and the --prefix
value
should specify an installation directory different than the one under which
the existing MySQL installation is located.
You can check the socket and port used by any currently-executing MySQL server with this command:
shell> mysqladmin -h hostname --port=port_number variables
If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.
You should also edit the initialization script for your machine (probably
`mysql.server') to start and kill multiple mysqld
servers.
You don't have to recompile a new MySQL server just to start with
a different port and socket. You can change the port and socket to be used
by specifying them at runtime as options to safe_mysqld
:
shell> /path/to/safe_mysqld --socket=file_name --port=port_number
If you run the new server on the same database directory as another server
with logging enabled, you should also specify the name of the log files
to safe_mysqld
with --log
and --log-update
. Otherwise,
both servers may be trying to write to the same log file.
Warning: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!
If you want to use another database directory for the second server, you
can use the --datadir=path
option to safe_mysqld
.
When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:
--host 'hostname' --port=port_numer
or
[--host localhost] --socket=file_name
.
MYSQL_UNIX_PORT
and MYSQL_TCP_PORT
environment variables
to point to the Unix socket and TCP/IP port before you start your clients.
If you normally use a specific socket or port, you should place commands
to set these environment variables in your `.login' file.
See section 12.1 Overview of the different MySQL programs.