PostgreSQL数据库切割和组合字段函数

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.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.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。其功能与mysql中的group_concat函数比较类似，但也有不同，pg中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.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.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',',');
 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.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