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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