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| -rw-r--r-- | doc/src/sgml/syntax.sgml | 20 |
1 files changed, 17 insertions, 3 deletions
diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml index bf63425a719..0c1f2068b18 100644 --- a/doc/src/sgml/syntax.sgml +++ b/doc/src/sgml/syntax.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.147 2010/07/03 02:57:46 rhaas Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.147.2.1 2010/08/04 22:31:55 tgl Exp $ --> <chapter id="sql-syntax"> <title>SQL Syntax</title> @@ -1568,7 +1568,7 @@ sqrt(2) unspecified order. In many cases this does not matter; for example, <function>min</> produces the same result no matter what order it receives the inputs in. However, some aggregate functions - (such as <function>array_agg</> and <function>xmlagg</>) produce + (such as <function>array_agg</> and <function>string_agg</>) produce results that depend on the ordering of the input rows. When using such an aggregate, the optional <replaceable>order_by_clause</> can be used to specify the desired ordering. The <replaceable>order_by_clause</> @@ -1576,13 +1576,27 @@ sqrt(2) described in <xref linkend="queries-order">, except that its expressions are always just expressions and cannot be output-column names or numbers. For example: - <programlisting> SELECT array_agg(a ORDER BY b DESC) FROM table; </programlisting> </para> <para> + When dealing with multiple-argument aggregate functions, note that the + <literal>ORDER BY</> clause goes after all the aggregate arguments. + For example, this: +<programlisting> +SELECT string_agg(a, ',' ORDER BY a) FROM table; +</programlisting> + not this: +<programlisting> +SELECT string_agg(a ORDER BY a, ',') FROM table; -- not what you want +</programlisting> + The latter syntax will be accepted, but <literal>','</> will be + treated as a (useless) sort key. + </para> + + <para> If <literal>DISTINCT</> is specified in addition to an <replaceable>order_by_clause</>, then all the <literal>ORDER BY</> expressions must match regular arguments of the aggregate; that is, |