PostgreSQL 9.6.2 문서 | |||
---|---|---|---|

이전 | 위로 | 장 9. Functions and Operators | 다음 |

*Aggregate functions* compute a single result
from a set of input values. The built-in normal aggregate functions
are listed in
표 9-51 and
표 9-52.
The built-in ordered-set aggregate functions
are listed in 표 9-53 and
표 9-54. Grouping operations,
which are closely related to aggregate functions, are listed in
표 9-55.
The special syntax considerations for aggregate
functions are explained in 4.2.7절.
Consult 2.7절 for additional introductory
information.

**표 9-51. General-Purpose Aggregate Functions**

Function | Argument Type(s) | Return Type | Partial Mode | Description |
---|---|---|---|---|

`array_agg(`
| any non-array type | array of the argument type | No | input values, including nulls, concatenated into an array |

`array_agg(`
| any array type | same as argument data type | No | input arrays concatenated into array of one higher dimension (inputs must all have same dimensionality, and cannot be empty or NULL) |

`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
| Yes | the average (arithmetic mean) of all input values |

`bit_and(`
| smallint, int, bigint, or
bit
| same as argument data type | Yes | 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 | Yes | the bitwise OR of all non-null input values, or null if none |

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

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

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

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

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

`json_agg(`
| any
| json
| No | aggregates values as a JSON array |

`jsonb_agg(`
| any
| jsonb
| No | aggregates values as a JSON array |

`json_object_agg(`
| (any, any)
| json
| No | aggregates name/value pairs as a JSON object |

`jsonb_object_agg(`
| (any, any)
| jsonb
| No | aggregates name/value pairs as a JSON object |

`max(`
| any numeric, string, date/time, network, or enum type, or arrays of these types | same as argument type | Yes | maximum value of across all input
values
expression |

`min(`
| any numeric, string, date/time, network, or enum type, or arrays of these types | same as argument type | Yes | minimum value of across all input
values
expression |

` string_agg(`
| (text, text) or (bytea, bytea)
| same as argument types | No | input values concatenated into a string, separated by delimiter |

`sum(`
| smallint, int,
bigint, real, double
precision, numeric,
interval, or money
| bigint for smallint or
int arguments, numeric for
bigint arguments, otherwise the same as the
argument data type
| Yes | sum of across all input valuesexpression |

`xmlagg(`
| xml
| xml
| No | concatenation of XML values (see also 9.14.1.7절) |

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.

Aggregate functions which support *Partial Mode*
are eligible to participate in various optimizations, such as parallel
aggregation.

참고: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 subquery returns one row with a Boolean value. Thus the standard name cannot be given to these aggregates.

참고: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 require effort proportional to the size of the table: PostgreSQL will need to scan either the entire table or the entirety of an index which includes all rows in the table.

The aggregate functions `array_agg`

,
`json_agg`

, `jsonb_agg`

,
`json_object_agg`

, `jsonb_object_agg`

,
`string_agg`

,
and `xmlagg`

, as well as similar user-defined
aggregate functions, produce meaningfully different result values
depending on the order of the input values. This ordering is
unspecified by default, but can be controlled by writing an
`ORDER BY` clause within the aggregate call, as shown in
4.2.7절.
Alternatively, supplying the input values from a sorted subquery
will usually work. For example:

SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;

Beware that this approach can fail if the outer query level contains additional processing, such as a join, because that might cause the subquery's output to be reordered before the aggregate is computed.

표 9-52 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

**표 9-52. Aggregate Functions for Statistics**

Function | Argument Type | Return Type | Partial Mode | Description |
---|---|---|---|---|

`corr(`
| double precision
| double precision
| Yes | correlation coefficient |

`covar_pop(`
| double precision
| double precision
| Yes | population covariance |

`covar_samp(`
| double precision
| double precision
| Yes | sample covariance |

`regr_avgx(`
| double precision
| double precision
| Yes | average of the independent variable
(sum())/XN |

`regr_avgy(`
| double precision
| double precision
| Yes | average of the dependent variable
(sum())/YN |

`regr_count(`
| double precision
| bigint
| Yes | number of input rows in which both expressions are nonnull |

`regr_intercept(`
| double precision
| double precision
| Yes | y-intercept of the least-squares-fit linear equation
determined by the (, X) pairsY |

`regr_r2(`
| double precision
| double precision
| Yes | square of the correlation coefficient |

`regr_slope(`
| double precision
| double precision
| Yes | slope of the least-squares-fit linear equation determined
by the (,
X) pairsY |

`regr_sxx(`
| double precision
| double precision
| Yes | sum( ("sum of
squares" of the independent variable)^2) - sum(X)^2/XN |

`regr_sxy(`
| double precision
| double precision
| Yes | sum( ("sum of
products" of independent times dependent
variable)*X) - sum(Y) * sum(X)/YN |

`regr_syy(`
| double precision
| double precision
| Yes | sum( ("sum of
squares" of the dependent variable)^2) - sum(Y)^2/YN |

`stddev(`
| smallint, int,
bigint, real, double
precision, or numeric
| double precision for floating-point arguments,
otherwise numeric
| Yes | historical alias for `stddev_samp` |

`stddev_pop(`
| smallint, int,
bigint, real, double
precision, or numeric
| double precision for floating-point arguments,
otherwise numeric
| Yes | population standard deviation of the input values |

`stddev_samp(`
| smallint, int,
bigint, real, double
precision, or numeric
| double precision for floating-point arguments,
otherwise numeric
| Yes | sample standard deviation of the input values |

`variance` ()
expression | smallint, int,
bigint, real, double
precision, or numeric
| double precision for floating-point arguments,
otherwise numeric
| Yes | historical alias for `var_samp` |

`var_pop` ()
expression | smallint, int,
bigint, real, double
precision, or numeric
| double precision for floating-point arguments,
otherwise numeric
| Yes | population variance of the input values (square of the population standard deviation) |

`var_samp` ()
expression | smallint, int,
bigint, real, double
precision, or numeric
| double precision for floating-point arguments,
otherwise numeric
| Yes | sample variance of the input values (square of the sample standard deviation) |

표 9-53 shows some
aggregate functions that use the *ordered-set aggregate*
syntax. These functions are sometimes referred to as "inverse
distribution" functions.

**표 9-53. Ordered-Set Aggregate Functions**

Function | Direct Argument Type(s) | Aggregated Argument Type(s) | Return Type | Partial Mode | Description |
---|---|---|---|---|---|

`mode() WITHIN GROUP (ORDER BY `
| any sortable type | same as sort expression | No | returns the most frequent input value (arbitrarily choosing the first one if there are multiple equally-frequent results) | |

`percentile_cont(`
| double precision
| double precision or interval
| same as sort expression | No | continuous percentile: returns a value corresponding to the specified fraction in the ordering, interpolating between adjacent input items if needed |

`percentile_cont(`
| double precision[]
| double precision or interval
| array of sort expression's type | No | multiple continuous percentile: returns an array of results matching
the shape of the parameter, with each
non-null element replaced by the value corresponding to that percentile
fractions |

`percentile_disc(`
| double precision
| any sortable type | same as sort expression | No | discrete percentile: returns the first input value whose position in the ordering equals or exceeds the specified fraction |

`percentile_disc(`
| double precision[]
| any sortable type | array of sort expression's type | No | multiple discrete percentile: returns an array of results matching the
shape of the parameter, with each non-null
element replaced by the input value corresponding to that percentile
fractions |

All the aggregates listed in 표 9-53
ignore null values in their sorted input. For those that take
a ` fraction` parameter, the fraction value must be
between 0 and 1; an error is thrown if not. However, a null fraction value
simply produces a null result.

Each of the aggregates listed in
표 9-54 is associated with a
window function of the same name defined in
9.21절. In each case, the aggregate result
is the value that the associated window function would have
returned for the "hypothetical" row constructed from
` args`, if such a row had been added to the sorted
group of rows computed from the

**표 9-54. Hypothetical-Set Aggregate Functions**

Function | Direct Argument Type(s) | Aggregated Argument Type(s) | Return Type | Partial Mode | Description |
---|---|---|---|---|---|

`rank(`
| VARIADIC "any"
| VARIADIC "any"
| bigint
| No | rank of the hypothetical row, with gaps for duplicate rows |

`dense_rank(`
| VARIADIC "any"
| VARIADIC "any"
| bigint
| No | rank of the hypothetical row, without gaps |

`percent_rank(`
| VARIADIC "any"
| VARIADIC "any"
| double precision
| No | relative rank of the hypothetical row, ranging from 0 to 1 |

`cume_dist(`
| VARIADIC "any"
| VARIADIC "any"
| double precision
| No | relative rank of the hypothetical row, ranging from
1/ to 1
N |

For each of these hypothetical-set aggregates, the list of direct arguments
given in ` args` must match the number and types of
the aggregated arguments given in

**표 9-55. Grouping Operations**

Function | Return Type | Description |
---|---|---|

`GROUPING(`
| integer
| Integer bit mask indicating which arguments are not being included in the current grouping set |

Grouping operations are used in conjunction with grouping sets (see
7.2.4절) to distinguish result rows. The
arguments to the `GROUPING` operation are not actually evaluated,
but they must match exactly expressions given in the `GROUP BY`
clause of the associated query level. Bits are assigned with the rightmost
argument being the least-significant bit; each bit is 0 if the corresponding
expression is included in the grouping criteria of the grouping set generating
the result row, and 1 if it is not. For example:

=>SELECT * FROM items_sold;make | model | sales -------+-------+------- Foo | GT | 10 Foo | Tour | 20 Bar | City | 15 Bar | Sport | 5 (4 rows)=>SELECT make, model, GROUPING(make,model), sum(sales) FROM items_sold GROUP BY ROLLUP(make,model);make | model | grouping | sum -------+-------+----------+----- Foo | GT | 0 | 10 Foo | Tour | 0 | 20 Bar | City | 0 | 15 Bar | Sport | 0 | 5 Foo | | 1 | 30 Bar | | 1 | 20 | | 3 | 50 (7 rows)