SQL编程：group 不用合并结果字符串

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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