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 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。
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