Postgresql里面内置了很多的实用函数，下面介绍下组合和切割函数
环境：PostgreSQL 9.1.2
CENTOS 5.7 final
一.组合函数
1.concat
a.语法介绍
concat(str "any" [, str "any" [, ...]])
Concatenate all but first arguments with separators.
The first parameter is used as a separator.
NULL arguments are ignored.
b.实际例子:
postgres=# create table t_kenyon(id int,name varchar(10),remark text);
CREATE TABLE
postgres=# insert into t_kenyon values(1,'test','kenyon'),(2,'just','china'),(3,'iam','lovingU');
INSERT 0 3
postgres=# insert into t_kenyon values(4,'test',null);
INSERT 0 1
postgres=# insert into t_kenyon values(5,null,'adele');
INSERT 0 1
postgres=# select * from t_kenyon;
id | name | remark
----+------+---------
1 | test | kenyon
2 | just | china
3 | iam | lovingU
4 | test |
5 | | adele
(5 rows)
postgres=# select concat(id,name,remark) from t_kenyon;
concat
-------------
1testkenyon
2justchina
3iamlovingU
4test
5adele
(5 rows)
c.说明
concat函数纯粹是一个拼接函数，可以忽略null值拼接，拼接的值没有分隔符，如果需要分割符，则需要用下面的函数concat_ws。
2.concat_ws
a.语法介绍
concat_ws(sep text, str "any" [, str "any" [,...] ])
Concatenate all but first arguments with separators.
The first parameter is used as a separator.
NULL arguments are ignored.
b.实际应用
postgres=# select concat_ws(',',id,name,remark) from t_kenyon;
concat_ws
---------------
1,test,kenyon
2,just,china
3,iam,lovingU
4,test
5,adele
(5 rows)
postgres=# select concat_ws('_',id,name,remark) from t_kenyon;
concat_ws
---------------
1_test_kenyon
2_just_china
3_iam_lovingU
4_test
5_adele
(5 rows)
postgres=# select concat_ws('',id,name,remark) from t_kenyon;
concat_ws
-------------
1testkenyon
2justchina
3iamlovingU
4test
5adele
(5 rows)
postgres=# select concat_ws('^_*',id,name,remark) from t_kenyon;
concat_ws
-------------------
1^_*test^_*kenyon
2^_*just^_*china
3^_*iam^_*lovingU
4^_*test
5^_*adele
(5 rows)
c.说明 concat_ws函数比concat函数多了分隔符的功能，其实就是concat的升级版，假如分隔符为'',则取出来的结果和concat是一样的。concat_ws分隔符还支持多个字符作为分隔符的，日常用得更多的可能是||。
二、切割函数
1.split_part
a.语法介绍
split_part(string text, delimiter text, field int)
Split string on delimiter and return the given field (counting from one)
b.实际例子
postgres=# select split_part('abc~@~def~@~ghi','~@~', 2);
split_part
------------
def
(1 row)
postgres=# select split_part('now|year|month','|',3);
split_part
------------
month
(1 row)
c.说明
该函数对按分隔符去取某个特定位置上的值非常有效果
2.regexp_split_to_table
a.语法介绍
regexp_split_to_table(string text, pattern text [, flags text])
Split string using a POSIX regular expression as the delimiter.
b.使用例子
postgres=# SELECT regexp_split_to_table('kenyon,love,,china,!',',');
regexp_split_to_table
-----------------------
kenyon
love
china
!
(5 rows)
--按分割符切割
postgres=# SELECT regexp_split_to_table('kenyon, china loves',E'\\s');
regexp_split_to_table
-----------------------
kenyon,
china
loves
(3 rows)
--按字母切割
postgres=# SELECT regexp_split_to_table('kenyon,,china',E'\\s*');
regexp_split_to_table
-----------------------
k
e
n
y
o
n
,
,
c
h
i
n
a
(13 rows)
3.regexp_split_to_array
a.语法介绍
regexp_split_to_array(string text, pattern text [, flags text ])
Split string using a POSIX regular expression as the delimiter.
b.实际例子
postgres=# SELECT regexp_split_to_array('kenyon,love,,china,!',',');
regexp_split_to_array
--------------------------
{kenyon,love,"",china,!}
(1 row)
postgres=# SELECT regexp_split_to_array('kenyon,love,,china!','s*');
regexp_split_to_array
-----------------------------------------------
{k,e,n,y,o,n,",",l,o,v,e,",",",",c,h,i,n,a,!}
(1 row)
c.说明
上面用到的flag里的s*表示split all，E'\\s'表示转义空格
