| PostgreSQL 9.4.4 文档 | |||
|---|---|---|---|
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本节描述那些可能返回多于一行的函数。目前这个类中被使用最广泛的是级数生成函数， 如Table 9-54和Table 9-55所述。其他更特殊的集合返回函数在本手册的其他地方描述。 组合多集合返回函数的方法可见Section 7.2.1.4。
Table 9-54. 级数生成函数
| 函数 | 参数类型 | 返回类型 | 描述 |
|---|---|---|---|
generate_series(start, stop) | int或bigint | setof int或setof bigint（与参数类型相同） | 产生一系列值，从start到stop，步长为 1 |
generate_series(start, stop, step) | int或bigint | setof int或setof bigint（和参数类型相同） | 产生一系列值，从start到stop，步长为step |
generate_series(start, stop, step interval) | timestamp或timestamp with time zone | setof timestamp或setof timestamp with time zone（和参数类型相同） | 产生一系列值，从start到stop，步长为step |
当step为正时，如果start大于stop则返回零行。相反，当step为负时，如果start小于stop则返回零行。对于NULL输入也会返回零行。step为零是一个错误。下面是一些例子：
SELECT * FROM generate_series(2,4);
generate_series
-----------------
2
3
4
(3 rows)
SELECT * FROM generate_series(5,1,-2);
generate_series
-----------------
5
3
1
(3 rows)
SELECT * FROM generate_series(4,3);
generate_series
-----------------
(0 rows)
-- 这个例子依赖于日期+整数操作符
SELECT current_date + s.a AS dates FROM generate_series(0,14,7) AS s(a);
dates
------------
2004-02-05
2004-02-12
2004-02-19
(3 rows)
SELECT * FROM generate_series('2008-03-01 00:00'::timestamp,
'2008-03-04 12:00', '10 hours');
generate_series
---------------------
2008-03-01 00:00:00
2008-03-01 10:00:00
2008-03-01 20:00:00
2008-03-02 06:00:00
2008-03-02 16:00:00
2008-03-03 02:00:00
2008-03-03 12:00:00
2008-03-03 22:00:00
2008-03-04 08:00:00
(9 rows)
Table 9-55. 下标生成函数
| 函数 | 返回类型 | 描述 |
|---|---|---|
generate_subscripts(array anyarray, dim int) | setof int | 生成一个级数组成给定数组的下标。 |
generate_subscripts(array anyarray, dim int, reverse boolean) | setof int | 生成一个级数组成给定数组的下标。当reverse为真，级数以逆序返回。 |
generate_subscripts是一个快捷函数，它为给定数组的指定维度生成一组合法的下标。对于不具有请求维度的数组返回零行，对于 NULL 数组也返回零行（但是会对 NULL 数组元素返回合法的下标）。下面是一些例子：
-- 基本使用
SELECT generate_subscripts('{NULL,1,NULL,2}'::int[], 1) AS s;
s
---
1
2
3
4
(4 rows)
-- 表示一个数组，下标和被下标的值需要一个子查询
SELECT * FROM arrays;
a
--------------------
{-1,-2}
{100,200,300}
(2 rows)
SELECT a AS array, s AS subscript, a[s] AS value
FROM (SELECT generate_subscripts(a, 1) AS s, a FROM arrays) foo;
array | subscript | value
---------------+-----------+-------
{-1,-2} | 1 | -1
{-1,-2} | 2 | -2
{100,200,300} | 1 | 100
{100,200,300} | 2 | 200
{100,200,300} | 3 | 300
(5 rows)
-- 平面化一个 2D 数组
CREATE OR REPLACE FUNCTION unnest2(anyarray)
RETURNS SETOF anyelement AS $$
select $1[i][j]
from generate_subscripts($1,1) g1(i),
generate_subscripts($1,2) g2(j);
$$ LANGUAGE sql IMMUTABLE;
CREATE FUNCTION
SELECT * FROM unnest2(ARRAY[[1,2],[3,4]]);
unnest2
---------
1
2
3
4
(4 rows)
When a function in the FROM clause is suffixed
by WITH ORDINALITY, a bigint column is
appended to the output which starts from 1 and increments by 1 for each row
of the function's output. This is most useful in the case of set returning
functions such as unnest().
-- set returning function WITH ORDINALITY
SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
ls | n
-----------------+----
pg_serial | 1
pg_twophase | 2
postmaster.opts | 3
pg_notify | 4
postgresql.conf | 5
pg_tblspc | 6
logfile | 7
base | 8
postmaster.pid | 9
pg_ident.conf | 10
global | 11
pg_clog | 12
pg_snapshots | 13
pg_multixact | 14
PG_VERSION | 15
pg_xlog | 16
pg_hba.conf | 17
pg_stat_tmp | 18
pg_subtrans | 19
(19 rows)