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# 4.14. Aggregate Functions

Author: Written by Isaac Wilcox on 2000-06-16

Aggregate functions compute a single result value from a set of input values. The special syntax considerations for aggregate functions are explained in Section 1.3.5. Consult the PostgreSQL Tutorial for additional introductory information.

Table 4-30. Aggregate Functions

Function Description Notes
AVG(expression) the average (arithmetic mean) of all input values Finding the average value is available on the following data types: smallint, integer, bigint, real, double precision, numeric, interval. The result is of type numeric for any integer type input, double precision for floating-point input, otherwise the same as the input data type.
`count`(*) number of input values The return value is of type bigint.
`count`(expression) Counts the input values for which the value of expression is not NULL. The return value is of type bigint.
`max`(expression) the maximum value of expression across all input values Available for all numeric, string, and date/time types. The result has the same type as the input expression.
`min`(expression) the minimum value of expression across all input values Available for all numeric, string, and date/time types. The result has the same type as the input expression.
`stddev`(expression) the sample standard deviation of the input values Finding the standard deviation is available on the following data types: smallint, integer, bigint, real, double precision, numeric. The result is of type double precision for floating-point input, otherwise numeric.
`sum`(expression) sum of expression across all input values Summation is available on the following data types: smallint, integer, bigint, real, double precision, numeric, interval. The result is of type bigint for smallint or integer input, numeric for bigint input, double precision for floating-point input, otherwise the same as the input data type.
`variance`(expression) the sample variance of the input values The variance is the square of the standard deviation. The supported data types and result types are the same as for standard deviation.

It should be noted that except for `COUNT`, these functions return NULL when no rows are selected. In particular, `SUM` of no rows returns NULL, not zero as one might expect. `COALESCE` may be used to substitute zero for NULL when necessary.