MySql-怎么查询删除数据表重复记录_mysql

查询

SELECT     name, email, COUNT(*) FROM     users GROUP BY     name, email HAVING      COUNT(*) > 1

重点来了，查询容易，那应该如何删除重复记录呢？

演示数据
表结构：

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.CodeBit.cn  | |  2 | http://YITU.org        | |  3 | http://www.ShuoWen.org | |  4 | http://www.CodeBit.cn  | |  5 | http://www.ShuoWen.org | +----+------------------------+ 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：

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.CodeBit.cn  | |  2 | http://YITU.org        | |  3 | http://www.ShuoWen.org | +----+------------------------+ 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：

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://YITU.org        | |  4 | http://www.CodeBit.cn  | |  5 | http://www.ShuoWen.org | +----+------------------------+ 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：

mysql> SELECT a.*      -> FROM demo a, demo b     -> WHERE a.id > b.id     -> AND (a.site = b.site); +----+------------------------+ | id | site                   | +----+------------------------+ |  1 | http://www.CodeBit.cn  | |  3 | http://www.ShuoWen.org | +----+------------------------+ 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：

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.CodeBit.cn  | |  2 | http://YITU.org        | |  3 | http://www.ShuoWen.org | +----+------------------------+ 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：

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：

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.CodeBit.cn  | |  2 | http://YITU.org        | |  3 | http://www.ShuoWen.org | |  4 | http://www.CodeBit.cn  | |  5 | http://www.ShuoWen.org | +----+------------------------+ 5 rows in set (0.00 sec)

mysql> select * from demo_new order by id; +----+------------------------+ | id | site                   | +----+------------------------+ |  1 | http://www.CodeBit.cn  | |  2 | http://YITU.org        | |  3 | http://www.ShuoWen.org | +----+------------------------+ 3 rows in set (0.00 sec)

然后将原表备份，将新表重命名为当前表：

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.CodeBit.cn  | |  2 | http://YITU.org        | |  3 | http://www.ShuoWen.org | +----+------------------------+ 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！

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。