1、表结构

create table tt(id int,v varchar(30));

insert into tt values(1,'a'),(1,'b'),(2,'b'),(2,'c'),(2,'b'),(2,'a'),(3,'a');

mysql> select * from tt;

+------+------+

| id | v |

+------+------+

| 1 | a |

| 1 | b |

| 2 | b |

| 2 | c |

| 2 | b |

| 2 | a |

| 3 | a |

+------+------+

7 rows in set (0.05 sec)

2、问题

一般在合并字符串我们可以用concat，但是这个函数主要实现类似字符串相加的目的，而且是同一行中的字符串的合并。

如果现在要实现把不同行的字符串进行合并，要如何实现呢？

3、解决方法：

完全合并

mysql> select group_concat(v) from tt;

+-----------------+

| group_concat(v) |

+-----------------+

| a,b,b,c,b,a,a |

+-----------------+

1 row in set (0.05 sec)

分组合并

mysql> select id,group_concat(v) from tt group by id;

+------+-----------------+

| id | group_concat(v) |

+------+-----------------+

| 1 | a,b |

| 2 | b,c,b,a |

| 3 | a |

+------+-----------------+

3 rows in set (0.01 sec)

分组、去重合并

mysql> select id,group_concat(distinct v) from tt group by id;

+------+--------------------------+

| id | group_concat(distinct v) |

+------+--------------------------+

| 1 | a,b |

| 2 | b,c,a |

| 3 | a |

+------+--------------------------+

3 rows in set (0.03 sec)

分组、去重、排序合并

mysql> select id,group_concat(distinct v order by v) from tt group by id;

+------+-------------------------------------+

| id | group_concat(distinct v order by v) |

+------+-------------------------------------+

| 1 | a,b |

| 2 | a,b,c |

| 3 | a |

+------+-------------------------------------+

3 rows in set (0.00 sec)

自定义连接符

mysql> select id,group_concat(v separator ';') from tt group by id;

+------+-------------------------------+

| id | group_concat(v separator ';') |

+------+-------------------------------+

| 1 | a;b |

| 2 | b;c;b;a |

| 3 | a |

+------+-------------------------------+

3 rows in set (0.01 sec)

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