concat和concat_ws()区别及MySQL的几个实用字符串函数

1、Concat()函数
    1.1 MySQL的concat函数可以连接一个或者多个字符串
,
        mysql
select concat('10');
        +--------------+
        | 
concat('10') |
        +--------------+
        | 
10           |
        +--------------+
       
1 row in set (0.00 sec)

       
mysqlselect concat('11','22','33');
        +------------------------+
        | 
concat('11','22','33') |
        +------------------------+
        | 
112233                 |
        +------------------------+

       
1 row in set (0.00 sec)

       
而Oracle的concat函数只能连接两个字符串

        SQL
select concat('11','22'from dual;

   
1.2 MySQL的concat函数在连接字符串的时候,只要其中一个是NULL,那么将返回NULL

        mysql
select concat('11','22',null);
        +------------------------+
        | 
concat('11','22',null) |
        +------------------------+
        | 
NULL                   |
        +------------------------+
       
1 row in set (0.00 sec)

       
而Oracle的concat函数连接的时候,只要有一个字符串不是NULL,就不会返回NULL

        SQL
select concat('11',NULLfrom dual;
       
CONCAT
       
--
       
11

2、concat_ws
()函数表示concat with separator,即有分隔符的字符串连接
    如连接后以逗号分隔
        mysql
select concat_ws(',','11','22','33');

        +-------------------------------+
        | 
concat_ws(',','11','22','33') |
        +-------------------------------+
        | 
11,22,33                      |
        +-------------------------------+
       
1 row in set (0.00 sec)

   
和concat不同的是concat_ws函数在执行的时候,不会因为NULL值而返回NULL
        mysql
select concat_ws(',','11','22',NULL);
        +-------------------------------+
        | 
concat_ws(',','11','22',NULL) |
        +-------------------------------+
        | 
11,22                         |
        +-------------------------------+
       
1 row in set (0.00 sec)

3、group_concat()可用来行转列Oracle没有这样的函数

    完整的语法如下
    group_concat
([DISTINCT要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])
   
如下例子
    mysql
select from aa;

    +------+------+
    | 
id   name |
    +------+------+
    |    
10   |
    |    
20   |
    |    
20   |
    |    
20   |
    |    
200  |
    |    
500  |
    +------+------+
   
6 rows in set (0.00 sec)
   3.1 以id分组,把name字段的值打印在一行,逗号分隔(默认)
       
mysqlselect id,group_concat(namefrom aa group by id;
        +------+--------------------+
        | 
id   group_concat(name) |
        +------+--------------------+
        |    
10,20,20           |
        |    
20                 |
        |    
200,500            |
        +------+--------------------+

       
3 rows in set (0.00 sec)

   
3.2 以id分组,把name字段的值打印在一行,分号分隔
        mysql
select id,group_concat(name separator ';'from aa group by id;
        +------+----------------------------------+
        | 
id   group_concat(name separator ';') |
        +------+----------------------------------+
        |    
10;20;20                         |
        |    
20                               |
        |    
200;500                          |
        +------+----------------------------------+

       
3 rows in set (0.00 sec)

   
3.3 以id分组,把去冗余的name字段的值打印在一行,逗号分隔

        mysql
select id,group_concat(distinct namefrom aa group by id;

        +------+-----------------------------+
        | 
id   group_concat(distinct name) |
        +------+-----------------------------+
        |    
10,20                       |
        |    
20                          |
        |    
200,500                     |
        +------+-----------------------------+

       
3 rows in set (0.00 sec)

   
3.4 以id分组,把name字段的值打印在一行,逗号分隔,以name排倒序

        mysql
select id,group_concat(name order by name descfrom aa group by id;

        +------+---------------------------------------+
        | 
id   group_concat(name order by name desc) |
        +------+---------------------------------------+
        |    
20,20,10                              |
        |    
20                                    |
        |    
500,200                               |
        +------+---------------------------------------+

       
3 rows in set (0.00 sec)

4、repeat()函数,用来复制字符串,如下'ab'表示要复制的字符串,2表示复制的份数

    mysql
select repeat('ab',2);

    +----------------+
    | 
repeat('ab',2) |
    +----------------+
    | 
abab           |
    +----------------+

   
1 row in set (0.00 sec)

   
又如
    mysql
select repeat('a',2);

    +---------------+
    | 
repeat('a',2) |
    +---------------+
    | 
aa            |
    +---------------+
   
1 row in set (0.00 sec)

转载 http://www.52crack.com/blog/?action=show&id=323

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