MYSQL中删除重复记录的方法 MySQLF#SQL
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
rminated 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 |
mysql> create temporary table tmp_wrap select * from users_groups group by uid having count(1) > 1 union all
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。