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Aggregate functions compute a single result from a set of input values. The built-in aggregate functions are listed in Table 9-42 and Table 9-43. The special syntax considerations for aggregate functions are explained in Section 4.2.7. Consult Section 2.7 for additional introductory information.
Table 9-42. General-Purpose Aggregate Functions
| Function | Argument Type | Return Type | Description |
|---|---|---|---|
array_agg(expression) |
any | array of the argument type | input values concatenated into an array |
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(*) |
bigint | number of input rows | |
count(expression) |
any | bigint | number of input rows 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 type | same as argument type | maximum value of expression across all input values |
min(expression) |
any array, numeric, string, or date/time type | same as argument type | minimum value of expression across all 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 |
xmlagg(expression) |
xml | xml | concatenation of XML values (see also Section 9.14.1.7) |
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, and array_agg returns
null rather than an empty array when there are no input rows. The
coalesce function can be used to
substitute zero or an empty array for null when necessary.
Note: Boolean aggregates
bool_andandbool_orcorrespond to standard SQL aggregateseveryandanyorsome. As foranyandsome, it seems that there is an ambiguity built into the standard syntax:SELECT b1 = ANY((SELECT b2 FROM t2 ...)) FROM t1 ...;Here
ANYcan be considered either as introducing a subquery, or as being an aggregate function, if the sub-select returns one row with a boolean value. Thus the standard name cannot be given to these aggregates.
Note: Users accustomed to working with other SQL database management systems might be disappointed by the performance of the
countaggregate 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.
The aggregate functions array_agg and xmlagg, as well as similar user-defined
aggregate functions, produce meaningfully different result values
depending on the order of the input values. In the current
implementation, the order of the input is in principle
unspecified. Supplying the input values from a sorted subquery
will usually work, however. For example:
SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
But this syntax is not allowed in the SQL standard, and is not portable to other database systems. A future version of PostgreSQL might provide an additional feature to control the order in a better-defined way (xmlagg(expr ORDER BY expr, expr, ...)).
Table 9-43 shows aggregate functions typically used in statistical analysis. (These are separated out merely to avoid cluttering the listing of more-commonly-used aggregates.) Where the description mentions N, it means the number of input rows for which all the input expressions are non-null. In all cases, null is returned if the computation is meaningless, for example when N is zero.
Table 9-43. Aggregate Functions for Statistics