diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/syntax.sgml | 27 |
1 files changed, 25 insertions, 2 deletions
diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml index b6ebb1cc434..a1c0767e4cc 100644 --- a/doc/src/sgml/syntax.sgml +++ b/doc/src/sgml/syntax.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.77 2003/03/25 16:15:38 petere Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.78 2003/06/06 15:04:01 tgl Exp $ --> <chapter id="sql-syntax"> @@ -1094,6 +1094,29 @@ sqrt(2) linkend="functions-aggregate">. Other aggregate functions may be added by the user. </para> + + <para> + An aggregate expression may only appear in the result list or + <literal>HAVING</> clause of a <command>SELECT</> command. + It is forbidden in other clauses, such as <literal>WHERE</>, + because those clauses are logically evaluated before the results + of aggregates are formed. + </para> + + <para> + When an aggregate expression appears in a subquery (see + <xref linkend="sql-syntax-scalar-subqueries"> and + <xref linkend="functions-subquery">), the aggregate is normally + evaluated over the rows of the subquery. But an exception occurs + if the aggregate's argument contains only outer-level variables: + the aggregate then belongs to the nearest such outer level, and is + evaluated over the rows of that query. The aggregate expression + as a whole is then an outer reference for the subquery it appears in, + and acts as a constant over any one evaluation of that subquery. + The restriction about + appearing only in the result list or <literal>HAVING</> clause + applies with respect to the query level that the aggregate belongs to. + </para> </sect2> <sect2 id="sql-syntax-type-casts"> @@ -1164,7 +1187,7 @@ CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable> </para> </sect2> - <sect2> + <sect2 id="sql-syntax-scalar-subqueries"> <title>Scalar Subqueries</title> <para> |