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分类: Mysql/postgreSQL

2008-05-15 15:40:24

MYSQL中删除重复记录的方法和代码

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

1

4 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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