1、表結構    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、問題一般在合併字串我們可以用concat，但是這個函式主要實現類似字串相加的目的，而且是同一行中的字串的合併。如果現在要實現把不同行的字串進行合併，要如何實現呢？3、解決方法：完全合併    mysql> select group_concat(v) from tt;    +-----------------+    | group_concat(v) |    +-----------------+    | a,b,b,c,b,a,a |    +-----------------+    1 row in set (0.05 sec)分組合並    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)分組、去重合並    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)分組、去重、排序合併    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)自定義連線符    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)---------------------