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Diffstat (limited to 'doc/src/sgml/syntax.sgml')
| -rw-r--r-- | doc/src/sgml/syntax.sgml | 53 |
1 files changed, 37 insertions, 16 deletions
diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml index a74bd08be11..beacf8fbdf0 100644 --- a/doc/src/sgml/syntax.sgml +++ b/doc/src/sgml/syntax.sgml @@ -642,15 +642,16 @@ CAST '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> <member><replaceable>a_expr</replaceable> <replaceable>right_unary_operator</replaceable></member> <member><replaceable>left_unary_operator</replaceable> <replaceable>a_expr</replaceable></member> <member>parameter</member> - <member>functional expressions</member> - <member>aggregate expressions</member> + <member>functional expression</member> + <member>aggregate expression</member> </simplelist> </para> <para> - We have already discussed constants and attributes. The two kinds of - operator expressions indicate respectively binary and left_unary - expressions. The following sections discuss the remaining options. + We have already discussed constants and attributes. The three kinds of + operator expressions indicate respectively binary (infix), right-unary + (suffix) and left-unary (prefix) operators. The following sections + discuss the remaining options. </para> <sect2> @@ -690,7 +691,7 @@ CREATE FUNCTION dept (name) enclosed in parentheses: <synopsis> -<replaceable>function</replaceable> (<replaceable>a_expr</replaceable> [, <replaceable>a_expr</replaceable> ) +<replaceable>function</replaceable> (<replaceable>a_expr</replaceable> [, <replaceable>a_expr</replaceable> ... ] ) </synopsis> </para> @@ -705,20 +706,40 @@ sqrt(emp.salary) </sect2> <sect2> - <title>Aggregate Expression</title> + <title>Aggregate Expressions</title> <para> - An <firstterm>aggregate expression</firstterm> - represents a simple aggregate (i.e., one that computes a single value) - or an aggregate function (i.e., one that computes a set of values). - The syntax is the following: + An <firstterm>aggregate expression</firstterm> represents the application + of an aggregate function across the rows selected by a query. + An aggregate function reduces multiple inputs to a single output value, + such as the sum or average of the inputs. + The syntax of an aggregate expression is one of the following: - <synopsis> -<replaceable>aggregate_name</replaceable> (<replaceable>attribute</replaceable>) - </synopsis> + <simplelist> + <member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member> + <member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member> + <member><replaceable>aggregate_name</replaceable> ( * )</member> + </simplelist> + + where <replaceable>aggregate_name</replaceable> is a previously defined + aggregate, and <replaceable>expression</replaceable> is any expression + that doesn't itself contain an aggregate expression. + </para> - where <replaceable>aggregate_name</replaceable> - must be a previously defined aggregate. + <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 + 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 + for the count() aggregate. + </para> + + <para> + For example, count(*) yields the total number of input rows; + count(f1) yields the number of input rows in which f1 is non-null; + count(distinct f1) yields the number of distinct non-null values of f1. </para> </sect2> |