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PostgreSQL 8.1.4 Documentation
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9.15. Aggregate Functions

Aggregate functions compute a single result value from a set of input values. Table 9-37 shows the built-in aggregate functions. The special syntax considerations for aggregate functions are explained in Section 4.2.7. Consult Section 2.7 for additional introductory information.

Table 9-37. Aggregate Functions

FunctionArgument TypeReturn TypeDescription
avg(expression) smallint, int, 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
bit_and(expression) smallint, int, bigint, or bit same as argument data type the bitwise AND of all non-null input values, or null if none
bit_or(expression) smallint, int, bigint, or bit same as argument data type the bitwise OR of all non-null input values, or null if none
bool_and(expression) bool bool true if all input values are true, otherwise false
bool_or(expression) bool bool true if at least one input value is true, otherwise false
count(*) bigintnumber of input values
count(expression)anybigint number of input values for which the value of expression is not null
every(expression) bool bool equivalent to bool_and
max(expression)any array, numeric, string, or date/time typesame as argument type maximum value of expression across all input values
min(expression)any array, numeric, string, or date/time typesame as argument type minimum value of expression across all input values
stddev(expression) smallint, int, bigint, real, double precision, or numeric double precision for floating-point arguments, otherwise numeric sample standard deviation of the input values
sum(expression) smallint, int, bigint, real, double precision, numeric, or interval bigint for smallint or int 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, int, 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 coalesce function may be used to substitute zero for null when necessary.

Note: Boolean aggregates bool_and and bool_or correspond to standard SQL aggregates every and any or some. As for any and some, it seems that there is an ambiguity built into the standard syntax: 

SELECT b1 = ANY((SELECT b2 FROM t2 ...)) FROM t1 ...;

Here ANY can be considered both as leading to a subquery or as an aggregate if the select expression returns 1 row. Thus the standard name cannot be given to these aggregates.

Note: Users accustomed to working with other SQL database management systems may be surprised by the performance of the count aggregate when it is applied to the entire table. A query like: 

SELECT count(*) FROM sometable;

will be executed by PostgreSQL using a sequential scan of the entire table.

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