MYSQL中删除重复记录的方法

gewanhong

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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 fieldsrminated 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
50114 rows in set (0.00 sec)
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
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
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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