mysql中删除重复记录,只保留一条

from http://hi.baidu.com/hexie007/blog/item/24946ea999d2e1f01f17a2b6.html

表结构如下:

mysql> desc test1;
+--------------+------------------+------+-----+---------+----------------+
| Field        | Type             | Null | Key | Default | Extra          |
+--------------+------------------+------+-----+---------+----------------+
| ID           | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| SENDERNAME   | varchar(32)      | YES  |     | NULL    |                |
| RECEIVERNAME | varchar(64)      | YES  |     | NULL    |                |
| SENDER       | varchar(64)      | NO   |     | NULL    |                |
| RECEIVER     | varchar(64)      | NO   |     | NULL    |                |
| SUBJECT      | varchar(512)     | NO   |     | NULL    |                |
| CONTENT      | text             | NO   |     | NULL    |                |
| PRIORITY     | int(11)          | NO   | MUL | NULL    |                |
| STATUS       | int(11)          | NO   | MUL | NULL    |                |
| CREATETIME   | datetime         | NO   |     | NULL    |                |
| SENDTIME     | datetime         | YES  |     | NULL    |                |
+--------------+------------------+------+-----+---------+----------------+

subject和RECEIVER   需要做uniq key,但设计时未做,后面的数据就有很多重复的记录。

1. 查询需要删除的记录,会保留一条记录。
select a.id,a.subject,a.RECEIVER from test1 a left join (select c.subject,c.RECEIVER ,max(c.id) as  bid from test1 c where status=0 GROUP BY RECEIVER,SUBJECT having count(1) >1) b on a.id< b.bid where  a.subject=b.subject and a.RECEIVER = b.RECEIVER and a.id < b.bid

2. 删除重复记录,只保留一条记录。注意,subject,RECEIVER 要索引,否则会很慢的。
delete a from test1 a, (select c.subject,c.RECEIVER ,max(c.id) as  bid from test1 c where status=0 GROUP BY RECEIVER,SUBJECT having count(1) >1) b where a.subject=b.subject and a.RECEIVER = b.RECEIVER and a.id < b.bid;

一个比较巧的方法 from http://www.php18.com/archives/Mysql_3012.html

Mysql删除重复的记录(只保留一条)

原理很简单:
把不重复的记录复制到一个新table,  之后删除旧table, 再把新table改回旧table的名不就行了?

create table new_table_name  select *, count(distinct title_name) from old_table_name  group by title_name

new_table_name就是新表名

old_table_name 为原表名

title_name 就是不想重复的字段

注意:这个方法创建的新表后面会多一个字段,删除即可




coded by nessus
发表评论?

0 条评论。

发表评论