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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.