-<!-- $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>
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</>
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,