这里有新鲜出炉的Mysql教程,程序狗速度看过来!
MySQL是一个开放源码的小型关联式数据库管理系统,开发者为瑞典MySQL AB公司。MySQL被广泛地应用在Internet上的中小型网站中。由于其体积小、速度快、总体拥有成本低,尤其是开放源码这一特点,许多中小型网站为了降低网站总体拥有成本而选择了MySQL作为网站数据库。
mysql中删除重复记录的方法有很多种,下面这篇文章主要给大家总结了在MySQL中查询、删除重复记录的方法大全,文中给出了详细的示例代码供大家参考学习,需要的朋友下面来一起看看吧。
前言
本文主要给大家介绍了关于MySQL中查询、删除重复记录的方法,分享出来供大家参考学习,下面来看看详细的介绍:
查找所有重复标题的记录:
- select title,count(*) as count from user_table group by title having count>1;
- SELECT * FROM t_info a WHERE((SELECT COUNT( * ) FROM t_info WHERE Title = a.Title) > 1) ORDER BY Title DESC
一、查找重复记录
1、查找全部重复记录
- SELECT * FROM t_info a WHERE((SELECT COUNT( * ) FROM t_info WHERE Title = a.Title) > 1) ORDER BY Title DESC
2、过滤重复记录(只显示一条)
- Select * From HZT Where ID In(Select Max(ID) From HZT Group By Title)
注:此处显示ID最大一条记录
二、删除重复记录
1、删除全部重复记录(慎用)
- Delete表Where重复字段In(Select重复字段From表Group By重复字段Having Count( * ) > 1)
2、保留一条(这个应该是大多数人所需要的 ^_^)
- Delete HZT Where ID Not In(Select Max(ID) From HZT Group By Title)
注:此处保留ID最大一条记录
三、举例
1、查找表中多余的重复记录,重复记录是根据单个字段(peopleId)来判断
- select * from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
2、删除表中多余的重复记录,重复记录是根据单个字段(peopleId)来判断,只留有rowid最小的记录
- delete from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) and rowid not in (select min(rowid) from people group by peopleId having count(peopleId) > 1)
3、查找表中多余的重复记录(多个字段)
- select * from vitae a where(a.peopleId, a.seq) in (select peopleId, seq from vitae group by peopleId, seq having count( * ) > 1)
4、删除表中多余的重复记录(多个字段),只留有rowid最小的记录
- delete from vitae a where(a.peopleId, a.seq) in (select peopleId, seq from vitae group by peopleId, seq having count( * ) > 1) and rowid not in (select min(rowid) from vitae group by peopleId, seq having count( * ) > 1)
5、查找表中多余的重复记录(多个字段),不包含rowid最小的记录
- select * from vitae a where(a.peopleId, a.seq) in (select peopleId, seq from vitae group by peopleId, seq having count( * ) > 1) and rowid not in (select min(rowid) from vitae group by peopleId, seq having count( * ) > 1)
四、补充
有两个以上的重复记录,一是完全重复的记录,也即所有字段均重复的记录,二是部分关键字段重复的记录,比如Name字段重复,而其他字段不一定重复或都重复可以忽略。
1、对于第一种重复,比较容易解决,使用
- select distinct * from tableName
就可以得到无重复记录的结果集。
如果该表需要删除重复的记录(重复记录保留1条),可以按以下方法删除
- select distinct * into #Tmp from tableName
- drop table tableName
- select * into tableName from #Tmp
- drop table #Tmp
发生这种重复的原因是表设计不周产生的,增加唯一索引列即可解决。
2、这类重复问题通常要求保留重复记录中的第一条记录,操作方法如下
假设有重复的字段为Name,Address,要求得到这两个字段唯一的结果集
- select identity(int, 1, 1) as autoID,
- *into#Tmp from tableName select min(autoID) as autoID into#Tmp2 from#Tmp group by Name,
- autoID select * from#Tmp where autoID in (select autoID from#tmp2)
总结
以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作能带来一定的帮助,如果有疑问大家可以留言交流,谢谢大家对PHPERZ的支持。
来源: http://www.phperz.com/article/17/1025/350474.html