mysql中删除重复记录sql语句

在sql中我们经常会碰到有重复的一些数据，下面我来介绍在mysql中删除重复记录的多种方法，有需要的朋友可参考参考。

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

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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.hzhuti.com  | |  2 | http://111cn.net        | |  3 | http://www.111cn.net | |  4 | http://www.hzhuti.com  | |  5 | http://www.111cn.net | +----+------------------------+ 5 rows in set (0.00 sec)   mysql> select * from demo_new order by id; +----+------------------------+ | id | site                   | +----+------------------------+ |  1 | http://www.hzhuti.com  | |  2 | http://111cn.net        | |  3 | http://www.111cn.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.hzhuti.com  | |  2 | http://111cn.net        | |  3 | http://www.111cn.net | +----+------------------------+ 3 rows in set (0.00 sec)