MYSQL中删除重复记录的方法

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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