4 rows in set (0.00 sec)

mysql> create temporary table tmp_wrap select * from users_groups group by uid having count(1) > 1 union all
select * from users_groups group by uid having count(1) = 1;
Query OK, 7 rows affected (0.11 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> truncate table users_groups;
Query OK, 14 rows affected (0.03 sec)

mysql> insert into users_groups select * from tmp_wrap;
Query OK, 7 rows affected (0.03 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> select * from users_groups;

query result(7 records)
id uid gid
1 11 502
2 107 502
3 100 503
4 110 501
5 112 501
6 104 502
9 102 501

mysql> drop table tmp_wrap;
　　Query OK, 0 rows affected (0.05 sec)
 

2、还有一个很精简的办法。

查找重复的，并且除掉最小的那个。

delete users_groups as a from users_groups as a,
(
select *,min(id) from users_groups group by uid having count(1) > 1
) as b
 where a.uid = b.uid and a.id > b.id;

(7 row(s)affected)
(0 ms taken)

query result(7 records)
id uid gid
1 11 502
2 107 502
3 100 503
4 110 501
5 112 501
6 104 502
9 102 501

3、现在来看一下这两个办法的效率。

运行一下以下SQL 语句

create index f_uid on users_groups(uid);
explain select * from users_groups group by uid having count(1) > 1 union all
select * from users_groups group by uid having count(1) = 1;

explain select * from  users_groups as a,
(
select *,min(id) from users_groups group by uid having count(1) > 1
) as b
 where a.uid = b.uid and a.id > b.id;

query result(3 records)
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY users_groups index (NULL) f_uid 4 (NULL) 14  
2 UNION users_groups index (NULL) f_uid 4 (NULL) 14  
(NULL) UNION RESULT <union1,2> ALL (NULL) (NULL) (NULL) (NULL) (NULL)  

query result(3 records)
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL (NULL) (NULL) (NULL) (NULL) 4  
1 PRIMARY a ref PRIMARY,f_uid f_uid 4 b.uid 1 Using where
2 DERIVED users_groups index (NULL) f_uid 4 (NULL) 14  

很明显的第二个比第一个扫描的函数要少。