表结构:mysql>descdemo;+-------+------------------+------+-----+---------+----------------+|Field|Type|Null|Key|D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。
当然,如果要避免重复记录,最好的办法还是不要插入重复数据,可以参考本站另外一篇文章:MySQL 当记录不存在时插入
重复记录