MySQL 数据库中删除重复记录的方法总结
jopen
11年前
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。