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PostgreSQL 8.4.22 Documentation | ||||
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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(` |
any | array of the argument type | input values concatenated into an array |

`avg(` |
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(` |
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(` |
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(` |
bool |
bool |
true if all input values are true, otherwise false |

`bool_or(` |
bool |
bool |
true if at least one input value is true, otherwise false |

`count(*)` |
bigint |
number of input rows | |

`count(` |
any | bigint |
number of input rows for which the value of
expression is not
null |

`every(` |
bool |
bool |
equivalent to `bool_and` |

`max(` |
any array, numeric, string, or date/time type | same as argument type | maximum value of expression across all input
values |

`min(` |
any array, numeric, string, or date/time type | same as argument type | minimum value of expression across all input
values |

`sum(` |
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(` |
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_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 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`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.

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