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MySQL数据库中删除重复记录方法总结

时间:2021-07-01 10:21:17 帮助过:47人阅读

本文章总结了关于在mysql数据库中各种删除重复记录的sql语句,下面我们用实例介绍了操作方法,大家可参考一下。

MYSQL数据库中,经常会遇到重复记录的情况,那么就需要SQL删除重复记录,下面为您列举了四种删除重复记录的方式,用于不同的情况,希望对您有所帮助。

1、查找表中多余的重复记录,重复记录是根据单个字段(peopleId)来判断

代码如下
select * from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)


2、SQL删除重复记录,重复记录是根据单个字段(peopleId)来判断,只留有rowid最小的记录

代码如下
delete from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1)


3、查找表中多余的重复记录(多个字段)

代码如下
select * from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)


4、删除表中多余的重复记录(多个字段),只留有rowid最小的记录

代码如下

delete from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1) and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)


演示数据

表结构:

代码如下

mysql> desc demo;

+-------+------------------+------+-----+---------+----------------+

| Field | Type | Null | Key | Default | Extra |

+-------+------------------+------+-----+---------+----------------+

| id | int(11) unsigned | NO | PRI | NULL | auto_increment |

| site | varchar(100) | NO | MUL | | |

+-------+------------------+------+-----+---------+----------------+

2 rows in set (0.00 sec)


数据:


mysql> select * from demo order by id;

+----+------------------------+

| id | site |

+----+------------------------+

| 1 | http://www.111cn.netn |

| 2 | http://安卓主题_www.hzhuti.com |

| 3 | http://www.zhutiy.com |

| 4 | http://www.111cn.netn |

| 5 | http://www.zhutiy.com |

+----+------------------------+

5 rows in set (0.00 sec)


当没有创建表或创建索引权限的时候,可以用下面的方法:

如果你要删除较旧的重复记录,可以使用下面的语句:

代码如下

mysql> delete from a

-> using demo as a, demo as b

-> where (a.id > b.id)

-> and (a.site = b.site);

Query OK, 2 rows affected (0.12 sec)

mysql> select * from demo order by id;

+----+------------------------+

| id | site |

+----+------------------------+

| 1 | http://www.111cn.netn |

| 2 | http://安卓主题_www.hzhuti.com |

| 3 | http://www.zhutiy.com |

+----+------------------------+

3 rows in set (0.00 sec)


如果你要删除较新的重复记录,可以使用下面的语句:

代码如下

mysql> delete from a

-> using demo as a, demo as b

-> where (a.id < b.id)

-> and (a.site = b.site);

Query OK, 2 rows affected (0.12 sec)

mysql> select * from demo order by id;

+----+------------------------+

| id | site |

+----+------------------------+

| 2 | http://安卓主题_www.hzhuti.com |

| 4 | http://www.111cn.netn |

| 5 | http://www.zhutiy.com |

+----+------------------------+

3 rows in set (0.00 sec)


你可以用下面的语句先确认将被删除的重复记录:

代码如下

mysql> SELECT a.*

-> FROM demo a, demo b

-> WHERE a.id > b.id

-> AND (a.site = b.site);

+----+------------------------+

| id | site |

+----+------------------------+

| 1 | http://www.111cn.netn |

| 3 | http://www.zhutiy.com |

+----+------------------------+

2 rows in set (0.00 sec)

如果有创建索引的权限,可以用下面的方法:

在表上创建唯一键索引:

代码如下

mysql> alter ignore table demo add unique index ukey (site);

Query OK, 5 rows affected (0.46 sec)

Records: 5 Duplicates: 2 Warnings: 0

mysql> select * from demo order by id;

+----+------------------------+

| id | site |

+----+------------------------+

| 1 | http://www.111cn.netn |

| 2 | http://安卓主题_www.hzhuti.com |

| 3 | http://www.zhutiy.com |

+----+------------------------+

3 rows in set (0.00 sec)


重复记录被删除后,如果需要,可以删除索引:

代码如下

mysql> alter table demo drop index ukey;

Query OK, 3 rows affected (0.37 sec)

Records: 3 Duplicates: 0 Warnings: 0


如果有创建表的权限,可以用下面的方法:

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

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.111cn.netn |

| 2 | http://安卓主题_www.hzhuti.com |

| 3 | http://www.zhutiy.com |

| 4 | http://www.111cn.netn |

| 5 | http://www.zhutiy.com |

+----+------------------------+

5 rows in set (0.00 sec)

mysql> select * from demo_new order by id;

+----+------------------------+

| id | site |

+----+------------------------+

| 1 | http://www.111cn.netn |

| 2 | http://安卓主题_www.hzhuti.com |

| 3 | http://www.zhutiy.com |

+----+------------------------+

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

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.111cn.netn |

| 2 | http://安卓主题_www.hzhuti.com |

| 3 | http://www.zhutiy.com |

+----+------------------------+

3 rows in set (0.00 sec)


注意:使用这种方式创建的表会丢失原表的索引信息!

代码如下

mysql> desc demo;

+-------+------------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+-------+------------------+------+-----+---------+-------+

| id | int(11) unsigned | NO | | 0 | |

| site | varchar(100) | NO | | | |

+-------+------------------+------+-----+---------+-------+

2 rows in set (0.00 sec)


如果要保持和原表信息一致,你可以使用 show create table demo; 来查看原表的创建语句,然后使用原表的创建语句创建新表,接着使用 insert … select 语句插入数据,再重命名表即可。


实例

今天无意导入几个测试数据,发现测试数据中,有很多数据记录是相同的,我现在就想删除掉这些数据,
如何查询数据相同记录呢?这个好说,以下语句就可以查看相同记录的了:

代码如下
SELECT COUNT(*) AS c, key_word FROM search_keywrod GROUP BY key_word HAVING c > 1

其中 HAVING c >1 代表相同记录数就有相同的了。

查询相同的是比较容易实现的了,但是想要删除这些重复的,估计就比较麻烦的了,因为你是要删除自身表里的记录,有些朋友可能就会使用到临时表,把相同需要删除的记录,导到时这个临时表,然后再通过临时表来删除主表。或者写一个临时程序,删除掉其中的一条重复记录。

以上二个方法删除重复记录,最大的麻烦就是操作繁琐。而我们今天介绍的就是利用MYSQL自身的语句,不创建临时表,不写程序来删除掉自身的重复记录。请看以下SQL语句:

代码如下
SELECT t1.id, t1.key_word
FROM search_keywrod t1, (
SELECT key_word, MIN(id) AS minid
FROM search_keywrod
GROUP BY key_word HAVING COUNT(key_word) > 1
)t2
WHERE t1.key_word = t2.key_word AND t1.id = t2.minid

这条语句就跟我们第一条语句是一样的功能,但是这条语句好处就是MIN(id),可以控制是删除大的id重复记录(MAX),还是删除小的id重复记录(MIN)。

OK,经过改良的语句实现了查询,现在就可以利用DELETE FROM语句来删除了。

代码如下

DELETE FROM search_keywrod WHERE id IN (SELECT id FROM (

SELECT t1.id
FROM search_keywrod t1, (
SELECT key_word, MIN(id) AS minid FROM search_keywrod
GROUP BY key_word HAVING COUNT(key_word) > 1
)t2
WHERE t1.key_word = t2.key_word AND t1.id = t2.minid

)t3)

执行时,请多执行几次,因为每次删除时,只删除掉重复的一次记录,如果你一条记录重复五次,那你就要执行五次的了。所以多执行几次,直到没有可删除的记录了,这样你直接一条语句删除掉重复的mysql记录功能就实现了

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