MYSQL中重複資料刪除記錄的方法

在實際應用中，很可能會碰到一些需要刪除某些欄位的重複記錄，我現在把我能想到的寫下來，望高手們補充。1、
具體實現如下：
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內容：
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)
根據一位兄弟的建議修改。
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、還有一個很精簡的辦法。尋找重複的，並且除掉最小的那個。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、現在來看一下這兩個辦法的效率。運行一下以下SQL 陳述式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

很明顯的第二個比第一個掃描的函數要少。

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