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)iduidgid11150221075023100503411050151125016104502710050281005019102501101045021110050212100501131025011411050114 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)iduidgid111502210750231005034110501511250161045029102501

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)iduidgid1115022107502310050341105015112501610450291025013、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)idselect_typetabletypepossible_keyskeykey_lenrefrowsExtra1PRIMARYusers_groupsindex(NULL)f_uid4(NULL)14?2UNIONusers_groupsindex(NULL)f_uid4(NULL)14?(NULL)UNION RESULT<union1,2>ALL(NULL)(NULL)(NULL)(NULL)(NULL)??query result(3 records)idselect_typetabletypepossible_keyskeykey_lenrefrowsExtra1PRIMARY<derived2>ALL(NULL)(NULL)(NULL)(NULL)4?1PRIMARYarefPRIMARY,f_uidf_uid4b.uid1Using where2DERIVEDusers_groupsindex(NULL)f_uid4(NULL)14???很明显的第二个比第一个扫描的函数要少。