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6.14. Aggregate Functions

Aggregate functions compute a single result value from a set of input values. Table 6-33 show the built-in aggregate functions. The special syntax considerations for aggregate functions are explained in Section 1.2.5. Consult the PostgreSQL 7.3.21 Tutorial for additional introductory information.

Table 6-33. Aggregate Functions

Function	Argument Type	Return Type	Description
`avg(expression)`	`smallint`, `integer`, `bigint`, `real`, `double precision`, `numeric`, or `interval`.	`numeric` for any integer type argument, `double precision` for a floating-point argument, otherwise the same as the argument data type	the average (arithmetic mean) of all input values
`count(*)`		`bigint`	number of input values
`count(expression)`	any	`bigint`	number of input values for which the value of `expression` is not null
`max(expression)`	any numeric, string, or date/time type	same as argument type	maximum value of `expression` across all input values
`min(expression)`	any numeric, string, or date/time type	same as argument type	minimum value of `expression` across all input values
`stddev(expression)`	`smallint`, `integer`, `bigint`, `real`, `double precision`, or `numeric`.	`double precision` for floating-point arguments, otherwise `numeric`.	sample standard deviation of the input values
`sum(expression)`	`smallint`, `integer`, `bigint`, `real`, `double precision`, `numeric`, or `interval`	`bigint` for `smallint` or `integer` arguments, `numeric` for `bigint` arguments, `double precision` for floating-point arguments, otherwise the same as the argument data type	sum of `expression` across all input values
`variance`(`expression`)	`smallint`, `integer`, `bigint`, `real`, `double precision`, or `numeric`.	`double precision` for floating-point arguments, otherwise `numeric`.	sample variance of the input values (square of the sample standard deviation)

It should be noted that except for count, these functions return a null value when no rows are selected. In particular, sum of no rows returns null, not zero as one might expect. The function coalesce may be used to substitute zero for null when necessary.

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