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mysql中删除重复记录sql语句

时间：2013-01-25 编辑：简简单单 来源：一聚教程网

删除重复记录方法一：

1. 新建一个临时表

 代码如下 复制代码

create table tmp as select * from youtable group by  name（name为不希望有重复的列）

2. 删除原来的表

 代码如下 复制代码

drop table youtable

3. 重命名表

 代码如下 复制代码

alter table tmp rename youtable

但是这个方法有个问题，由临时表转变过来的最终表，其表结构会和原来的不一致，需要手工更改。这个问题，待解决。
删除重复记录方法二：

1. 新建一个临时表

 代码如下 复制代码

CREATE TABLE tmp AS SELECT * FROM youtable GROUP BY name（name为不希望有重复的列）

2. 清空原来的表

 代码如下 复制代码

TRUNCATE TABLE youtable

3. 把临时表插入到youtable

 代码如下 复制代码

INSERT INTO tablename SELECT  * FROM temp

4. 删除临时表

 代码如下 复制代码

DROP TABLE  temp

删除重复记录方法三：

 代码如下 复制代码

delete table where ID not in(select min(ID) from table group by name(name:重复的字段))

删除重复记录方法四：


具体实现如下：

 代码如下 复制代码

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。

 代码如下 复制代码

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。

 代码如下 复制代码

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

 代码如下 复制代码

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。

当没有创建表或创建索引权限的时候

创建一个新表，然后将原表中不重复的数据插入新表：

 代码如下 复制代码

mysql> create table demo_new as select * from demo group by site;
Query OK, 3 rows affected (0.19 sec)
Records: 3  Duplicates: 0  Warnings: 0
 
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo           |
| demo_new       |
+----------------+
2 rows in set (0.00 sec)
 
mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.111com.net  |
|  2 | http://111com.net        |
|  3 | http://www.111com.net |
|  4 | http://www.111com.net  |
|  5 | http://www.111com.net |
+----+------------------------+
5 rows in set (0.00 sec)
 
mysql> select * from demo_new order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.111com.net  |
|  2 | http://111com.net        |
|  3 | http://www.111com.net |
+----+------------------------+
3 rows in set (0.00 sec)

然后将原表备份，将新表重命名为当前表：

 代码如下 复制代码

mysql> rename table demo to demo_old, demo_new to demo;
Query OK, 0 rows affected (0.04 sec)
 
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo           |
| demo_old       |
+----------------+
2 rows in set (0.00 sec)
 
mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.111com.net  |
|  2 | http://111com.net        |
|  3 | http://www.111com.net |
+----+------------------------+
3 rows in set (0.00 sec)

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