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QSqlQuery Class Reference
[sql module]

The QSqlQuery class provides a means of executing and manipulating SQL statements. More...

#include <qsqlquery.h>

Inherited by QSqlCursor.

List of all member functions.

Public Members

Protected Members

Detailed Description

The QSqlQuery class provides a means of executing and manipulating SQL statements.

QSqlQuery encapsulates the functionality involved in creating, navigating and retrieving data from SQL queries which are executed on a QSqlDatabase. It can be used to execute DML (data manipulation language) statements, e.g. SELECT, INSERT, UPDATE and DELETE, and also DDL (data definition language) statements, e.g. CREATE TABLE. It can also be used to execute database-specific commands which are not standard SQL (e.g. SET DATESTYLE=ISO for PostgreSQL).

Successfully executed SQL statements set the query's state to active (isActive() returns TRUE); otherwise the query's state is set to inactive. In either case, when executing a new SQL statement, the query is positioned on an invalid record; an active query must be navigated to a valid record (so that isValid() returns TRUE) before values can be retrieved.

Navigating records is performed with the following functions:

These functions allow the programmer to move forward, backward or arbitrarily through the records returned by the query. Once an active query is positioned on a valid record, data can be retrieved using value(). All data is transferred from the SQL backend using QVariants.

For example: 

    QSqlQuery query( "SELECT name FROM customer" );
    while ( query.next() ) {
        QString name = query.value(0).toString();
        doSomething( name );
    }

To access the data returned by a query, use the value() method. Each field in the data returned by a SELECT statement is accessed by passing the field's position in the statement, starting from 0. For the sake of efficiency there are no methods to access a field by name. (The QSqlCursor class provides a higher level interface for that generates SQL automatically and through which fields are accessed by name.)

QSqlQuery supports prepared query execution and binding of parameter values to placeholders. Note that only input values may be bound. Be aware that not all databases support these features. Currently only the Oracle and ODBC drivers have proper prepared query support, but the rest of the drivers support this by emulating the missing features (the placeholders are simply replaced with the actual value when the query is executed). It is also important to know that different databases use different placeholder marks for value binding. Oracle uses a : character followed by a placeholder name, while ODBC only uses a ? character to identify a placeholder. In an attempt to make this database independant we substitute the markers if you try to use ODBC markers in a query to an Oracle database and vice versa. Note that you can't mix the different bind styles by binding some values using named placeholders and some using positional placeholders.

Example: 

    // Named binding using named placeholders
    QSqlQuery q;
    q.prepare( "insert into mytable (id, name, lastname) values (:id, :name, :lname)" );
    q.bindValue( ":id", 0 );
    q.bindValue( ":name", "Testname" );
    q.bindValue( ":lname", "Lastname" );
    q.exec();

    // Positional binding using named placeholders
    QSqlQuery q;
    q.prepare( "insert into mytable (id, name, lastname) values (:id, :name, :lname)" );
    q.bindValue( 0, 0 );
    q.bindValue( 1, "Testname" );
    q.bindValue( 2, "Lastname" );
    q.exec();

    // Binding values using positional placeholders
    QSqlQuery q;
    q.prepare( "insert into mytable (id, name, lastname) values (?, ?, ?)" );
    q.bindValue( 0, 0 );
    q.bindValue( 1, "Testname" );
    q.bindValue( 2, "Lastname" );
    q.exec();
    
    // or alternatively
    q.prepare( "insert into mytable (id, name, lastname) values (?, ?, ?)" );
    q.addBindValue( 0 );
    q.addBindValue( "Testname" );
    q.addBindValue( "Lastname" );
    q.exec();

See also QSqlDatabase, QSqlCursor, QVariant, and Database Classes.

Member Function Documentation

QSqlQuery::QSqlQuery ( QSqlResult * r )

Creates a QSqlQuery object which uses the QSqlResult r to communicate with a database.

QSqlQuery::QSqlQuery ( const QString & query = QString::null, QSqlDatabase * db = 0 )

Creates a QSqlQuery object using the SQL query and the database db. If db is 0, (the default), the application's default database is used.

See also QSqlDatabase.

QSqlQuery::QSqlQuery ( const QSqlQuery & other )

Constructs a copy of other.

QSqlQuery::~QSqlQuery () [virtual]

Destroys the object and frees any allocated resources.

void QSqlQuery::addBindValue ( const QVariant & val )

Adds the value val to the list of values when using positional value binding. The order of the addBindValue() calls determines which placeholder a value will be bound to in the prepared query. Placeholder values are cleared after the query has been executed.

See also bindValue(), prepare(), and exec().

void QSqlQuery::afterSeek () [virtual protected]

Protected virtual function called after the internal record pointer is moved to a new record. The default implementation does nothing.

int QSqlQuery::at () const

Returns the current internal position of the query. The first record is at position zero. If the position is invalid, a QSql::Location will be returned indicating the invalid position.

See also isValid().

Example: sql/overview/navigating/main.cpp.

void QSqlQuery::beforeSeek () [virtual protected]

Protected virtual function called before the internal record pointer is moved to a new record. The default implementation does nothing.

void QSqlQuery::bindValue ( const QString & placeholder, const QVariant & val )

Set the placeholder placeholder to be bound to value val in the prepared statement. Note that the placeholder mark (e.g :) should be included when specifying the placeholder name. Placeholder values are cleared after the query has been executed.

See also addBindValue(), prepare(), and exec().

void QSqlQuery::bindValue ( int pos, const QVariant & val )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Set the placeholder in position pos to be bound to value val in the prepared statement. Field numbering starts at 0. Placeholder values are cleared after the query has been executed.

See also addBindValue(), prepare(), and exec().

const QSqlDriver * QSqlQuery::driver () const

Returns the database driver associated with the query.

bool QSqlQuery::exec ( const QString & query ) [virtual]

Executes the SQL in query. Returns TRUE and sets the query state to active if the query was successful; otherwise returns FALSE and sets the query state to inactive. The query string must use syntax appropriate for the SQL database being queried, for example, standard SQL.

After the query is executed, the query is positioned on an invalid record, and must be navigated to a valid record before data values can be retrieved, e.g. using next().

Note that the last error for this query is reset when exec() is called.

See also isActive(), isValid(), next(), prev(), first(), last(), and seek().

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, and sql/overview/basicdatamanip/main.cpp.

bool QSqlQuery::exec ()

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Executes a previously prepared SQL query. Returns TRUE if the query is executed successfully; otherwise returns FALSE.

See also prepare(), bindValue(), and addBindValue().

bool QSqlQuery::first () [virtual]

Retrieves the first record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE. Returns TRUE if successful. If unsuccessful the query position is set to an invalid position and FALSE is returned.

Example: sql/overview/navigating/main.cpp.

bool QSqlQuery::isActive () const

Returns TRUE if the query is currently active; otherwise returns FALSE.

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/basicdatamanip/main.cpp, sql/overview/navigating/main.cpp, and sql/overview/retrieve1/main.cpp.

bool QSqlQuery::isForwardOnly () const

Returns TRUE when you can only scroll forward through a result set otherwise FALSE

bool QSqlQuery::isNull ( int field ) const

Returns TRUE if the query is active and positioned on a valid record and the field is NULL; otherwise returns FALSE. Note that, for some drivers, isNull() will not return accurate information until after an attempt is made to retrieve data.

See also isActive(), isValid(), and value().

bool QSqlQuery::isSelect () const

Returns TRUE if the current query is a SELECT statement; otherwise returns FALSE.

bool QSqlQuery::isValid () const

Returns TRUE if the query is currently positioned on a valid record; otherwise returns FALSE.

bool QSqlQuery::last () [virtual]

Retrieves the last record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE. Returns TRUE if successful. If unsuccessful the query position is set to an invalid position and FALSE is returned.

Example: sql/overview/navigating/main.cpp.

QSqlError QSqlQuery::lastError () const

Returns error information about the last error (if any) that occurred.

See also QSqlError.

QString QSqlQuery::lastQuery () const

Returns the text of the current query being used, or QString::null if there is no current query text.

bool QSqlQuery::next () [virtual]

Retrieves the next record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE.

The following rules apply:

If the record could not be retrieved, the result is positioned after the last record and FALSE is returned. If the record is successfully retrieved, TRUE is returned.

See also at() and isValid().

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/delete/main.cpp, sql/overview/order1/main.cpp, sql/overview/retrieve1/main.cpp, sql/overview/subclass4/main.cpp, and sql/overview/subclass5/main.cpp.

int QSqlQuery::numRowsAffected () const

Returns the number of rows affected by the result's SQL statement, or -1 if it cannot be determined. Note that for SELECT statements, this value will be the same as size(). If the query is not active (isActive() returns FALSE), -1 is returned.

See also size() and QSqlDriver::hasFeature().

Examples: sql/overview/basicbrowsing2/main.cpp and sql/overview/basicdatamanip/main.cpp.

QSqlQuery & QSqlQuery::operator= ( const QSqlQuery & other )

Assigns other to the query.

bool QSqlQuery::prepare ( const QString & query )

Prepares the SQL query query for execution. The query may contain placeholders for binding values. Note that placeholder markers are usually database dependent.

See also exec(), bindValue(), and addBindValue().

bool QSqlQuery::prev () [virtual]

Retrieves the previous record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE.

The following rules apply:

If the record could not be retrieved, the result is positioned before the first record and FALSE is returned. If the record is successfully retrieved, TRUE is returned.

See also at().

const QSqlResult * QSqlQuery::result () const

Returns the result associated with the query.

bool QSqlQuery::seek ( int i, bool relative = FALSE ) [virtual]

Retrieves the record at position (offset) i, if available, and positions the query on the retrieved record. The first record is at position 0. Note that the query must be in an active state and isSelect() must return TRUE before calling this function.

If relative is FALSE (the default), the following rules apply:

If relative is TRUE, the following rules apply:

Example: sql/overview/navigating/main.cpp.

void QSqlQuery::setForwardOnly ( bool forward )

Sets forward only mode to forward. If forward is TRUE only next() and seek() with positive values are allowed for navigating the results. Forward only mode needs far less memory since results do not have to be cached. Forward only mode is off by default. Note that it is not possible to use forward only mode with data aware widgets like QDataTable since they need to be able to scroll backward.

See also next() and seek().

int QSqlQuery::size () const

Returns the size of the result, (number of rows returned), or -1 if the size cannot be determined or the database does not support reporting information about query sizes. Note that for non-SELECT statements (isSelect() returns FALSE), size() will return -1. If the query is not active (isActive() returns FALSE), -1 is returned.

To determine the number of rows affected by a non-SELECT statement, use numRowsAffected().

See also isActive(), numRowsAffected(), and QSqlDriver::hasFeature().

Example: sql/overview/navigating/main.cpp.

QVariant QSqlQuery::value ( int i ) const [virtual]

Returns the value of the i-th field in the query (zero based).

The fields are numbered from left to right using the text of the SELECT statement, e.g. in "SELECT forename, surname FROM people", field 0 is forename and field 1 is surname. Using SELECT * is not recommended because the order of the fields in the query is undefined.

An invalid QVariant is returned if field i does not exist, if the query is inactive, or if the query is positioned on an invalid record.

See also prev(), next(), first(), last(), seek(), isActive(), and isValid().

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/retrieve1/main.cpp, sql/overview/subclass3/main.cpp, sql/overview/subclass4/main.cpp, sql/overview/subclass5/main.cpp, and sql/overview/table4/main.cpp.

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