MYSQL中删除重复记录的方法

太阳不下山

　　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。