<simplelist>

<member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member>

<member><replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)</member>

<member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member>

<member><replaceable>aggregate_name</replaceable> ( * )</member>

</simplelist>

<para>

The first form of aggregate expression invokes the aggregate across all

input rows for which the given expression yields a non-null value.

The second form invokes the aggregate for all distinct non-null values

The second form is the same as the first, since ALL is the default.

The third form invokes the aggregate for all distinct non-null values

of the expression found in the input rows. The last form invokes the

aggregate once for each input row regardless of null or non-null values;

since no particular input value is specified, it is generally only useful