在实际应用中，很可能会碰到一些需要删除某些字段的重复记录，我现在把我能想到的写下来，望高手们补充。

1、

具体实现如下：

Table         Create Table                                            

------------  --------------------------------------------------------

users_groups  CREATE TABLE `users_groups` (                           

                `id` int(10) unsigned NOT NULL AUTO_INCREMENT,        

                `uid` int(11) NOT NULL,                               

                `gid` int(11) NOT NULL,                               

                PRIMARY KEY (`id`)                                    

              ) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8  

users_groups.txt内容：

1,11,502

2,107,502

3,100,503

4,110,501

5,112,501

6,104,502

7,100,502

8,100,501

9,102,501

10,104,502

11,100,502

12,100,501

13,102,501

14,110,501

mysql> load data infile 'c:\\users_groups.txt' into table users_groups fields

terminated by ',' lines terminated by '\n';

Query OK, 14 rows affected (0.05 sec)

Records: 14  Deleted: 0  Skipped: 0  Warnings: 0

mysql> select * from users_groups;

query result(14 records)

id  uid gid

1   11  502

2   107 502

3   100 503

4   110 501

5   112 501

6   104 502

7   100 502

8   100 501

9   102 501

10  104 502

11  100 502

12  100 501

13  102 501

14  110 501

14 rows in set (0.00 sec)

根据一位兄弟的建议修改。

mysql> create temporary table tmp_wrap 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。

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