05/15/80	mrds.selection_expressions

Function:	 This describes, in brief, the MRDS database query language, normally
referred to as a selection expression.


Syntax:
-range (tuple_variable1 relation_name1) {... (tuple_variableN relation_nameN)}
-select tuple_variable1.attr_name1 {... tuple_variable1.attr_nameN}
				.
				.
        tuple_variableN.attr_name1 {... tuple_variableN.attr_name}
{-where {(((...} tuple_variable1.attr_name = tuple_variable2.attr_name { )  &
				.
				.
	      tuple_variableN-1.attr_name = tuple_variableN.attr_name) } }


Semantics:
The range clause specifies which relations in the database will be involved in
this query, and associates a tuple variable with each relation.  There may be
more than one tuple variable, if need be, per relation.

The select clause specifies which attributes, in which relations, are to be
selected, upon satisfaction of the where clause conditions.	 If only a
tuple_variable is given, then all attributes in the corresponding relation are
selected.

The where clause specifies under what conditions, data is to be selected from
the database.  It is more general than shown, the = could be any of =, ^=, <,
>, <=, >=.  The & connector could have been |.  A whole expression could have
been negated by ^(expression).  The comparisons can be against not only other
tuple variable qualified attributes, but constants and expressions involving
the arithmetic operators +, -, /, and *, as well as mrds builtin or user
defined functions.	Such expressions are enclosed in brackets, as in
[tuple_variable.attr_name + 2], and can involve only one tuple variable per
expression.  Values can be substitued in place of a constant in the selection
expression string, in the where clause comparisons, by using the symbol .V.  in
place of the constant, and then in the subroutine call, using a se_value in the
argument list.  Complicated where clauses can be built using parentheses for
ordering of the evaluation.

The where clause is optional, and if not given, all tuples in the range are
selected.

See the help file mrds.builtins.info.


Exceptions:  The range clause can have a .V.  argument substitution in place of
a relation name, in order to make use of a temporary relation index (not a
relation name).  The range clause may also have an option added after -range.
These options are -print_search_order (-pso), and -no_optimize (-no_ot).  They
can be used to determine or produce an optimal data access search procedure.

The select clause can have a -dup specified, to indicated that retrieved
duplicates are not to be eliminated, which can improve efficiency, but can not
be used with set operations.

The select clause can have an asterisk following a attribute name, when
defining a temporary relation, in order to specify a key attribute for the
temporary relation.

Individual selection expressions can be combined using set operations, for
retrieves, when the results of each are union compatible results.  The syntax
is (se) set_op (se).  Set_op may be one of -inter, -union, or -differ for set
intersection, union, or difference.  These set operations may be nested using
parenthesis.

When doing retrieves, the entire selection expression may be replaced by
"-another" to retrieve subsequent tuples satisfying the selection expression
that found the first tuple.

Similarily, "-current tuple_variable1.attr_name 1 ...
tuple_variable1.attr_nameN" can be used to reselect the same tuple previously
selected, and is useful in conjunction with modifies and deletions, done after
retrieves using -another.


                                          -----------------------------------------------------------


Historical Background

This edition of the Multics software materials and documentation is provided and donated
to Massachusetts Institute of Technology by Group BULL including BULL HN Information Systems Inc. 
as a contribution to computer science knowledge.  
This donation is made also to give evidence of the common contributions of Massachusetts Institute of Technology,
Bell Laboratories, General Electric, Honeywell Information Systems Inc., Honeywell BULL Inc., Groupe BULL
and BULL HN Information Systems Inc. to the development of this operating system. 
Multics development was initiated by Massachusetts Institute of Technology Project MAC (1963-1970),
renamed the MIT Laboratory for Computer Science and Artificial Intelligence in the mid 1970s, under the leadership
of Professor Fernando Jose Corbato. Users consider that Multics provided the best software architecture 
for managing computer hardware properly and for executing programs. Many subsequent operating systems 
incorporated Multics principles.
Multics was distributed in 1975 to 2000 by Group Bull in Europe , and in the U.S. by Bull HN Information Systems Inc., 
as successor in interest by change in name only to Honeywell Bull Inc. and Honeywell Information Systems Inc. .

                                          -----------------------------------------------------------

Permission to use, copy, modify, and distribute these programs and their documentation for any purpose and without
fee is hereby granted,provided that the below copyright notice and historical background appear in all copies
and that both the copyright notice and historical background and this permission notice appear in supporting
documentation, and that the names of MIT, HIS, BULL or BULL HN not be used in advertising or publicity pertaining
to distribution of the programs without specific prior written permission.
    Copyright 1972 by Massachusetts Institute of Technology and Honeywell Information Systems Inc.
    Copyright 2006 by BULL HN Information Systems Inc.
    Copyright 2006 by Bull SAS
    All Rights Reserved