查询

SELECT    name, email, COUNT(*)FROM    usersGROUP BY    name, emailHAVING     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 | CodeBit.cn  ||  2 | YITU.org        ||  3 | ShuoWen.org ||  4 | CodeBit.cn  ||  5 | 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 | CodeBit.cn  ||  2 | YITU.org        ||  3 | 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 | YITU.org        ||  4 | CodeBit.cn  ||  5 | 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 | CodeBit.cn  ||  3 | 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 | CodeBit.cn  ||  2 | YITU.org        ||  3 | 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 | CodeBit.cn  ||  2 | YITU.org        ||  3 | ShuoWen.org ||  4 | CodeBit.cn  ||  5 | ShuoWen.org |+----+------------------------+5 rows in set (0.00 sec)

mysql> select * from demo_new order by id;+----+------------------------+| id | site                   |+----+------------------------+|  1 | CodeBit.cn  ||  2 | YITU.org        ||  3 | 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 | CodeBit.cn  ||  2 | YITU.org        ||  3 | 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。