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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)

       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',NULL) from 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 |
   +------+------+
   |    1 | 10   |
   |    1 | 20   |
   |    1 | 20   |
   |    2 | 20   |
   |    3 | 200  |
   |    3 | 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)
       mysql> select id,group_concat(name) from aa group by id;
       +------+--------------------+
       | id   | group_concat(name) |
       +------+--------------------+
       |    1 | 10,20,20           |
       |    2 | 20                 |
       |    3 | 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 ';') |
       +------+----------------------------------+
       |    1 | 10;20;20                         |
       |    2 | 20                               |
       |    3 | 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 name) from aa group by id;
       +------+-----------------------------+
       | id   | group_concat(distinct name) |
       +------+-----------------------------+
       |    1 | 10,20                       |
       |    2 | 20                          |
       |    3 | 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 desc) from aa group by id;
       +------+---------------------------------------+
       | id   | group_concat(name order by name desc) |
       +------+---------------------------------------+
       |    1 | 20,20,10                              |
       |    2 | 20                                    |
       |    3 | 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)