An aggregate expression represents the application of an aggregate function across the rows selected by a query. An aggregate function takes in multiple inputs and returns a single output value, such as the sum or average of the inputs. The possible syntaxes for an aggregate function are as follows:

aggregate_name ( expression )

This function call invokes the aggregate across all input rows for which the given expression yields a non-NULL value. NOTE: An aggregate function may or may not ignore a NULL value, depending on its definition, but the standard aggregate functions are set to ignore NULLs.

aggregate_name (ALL expression )

This function call is in essence the same as the first syntax because it specifies ALL as the default.

aggregate_name (DISTINCT expression )

This invokes the aggregate function for all distinct (non-repeating) non-NULL values of the expression found in the input rows.

aggregate_name (*)

Here, the aggregate is invoked once for each input row, regardless if the value is NULL or non-NULL because there is no input value specified. This is generally used for the count() function.

Note Aggregate Syntax

Aggregate_name is a previously defined aggregate, and expression is any expression that does not contain an aggregate expression.