fangzhen

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

       mysql> select concat('11','22','33');
        +------------------------+
        | concat('11','22','33') |
        +------------------------+
        | 112233                 |
        +------------------------+

       1 row in set (0.00 sec)

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)
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)