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重复记录分为两种,第一种是完全重复的记录,也就是所有字段均重复的记录,第二种是部分关键字段重复的记录,例如Name字段重复,而其它字段不一定重复或都重复。

1、第一种重复很容易解决,不同数据库环境下方法相似:

Mysql

create table tmp select distinct * from tableName;

drop table tableName;

create table tableName select * from tmp;

drop table tmp;


SQL Server

select distinct * into #Tmp from tableName;

drop table tableName;

select * into tableName from #Tmp;

drop table #Tmp;


Oracle

create table tmp as select distinct * from tableName;

drop table tableName;

create table tableName as select * from tmp;

drop table tmp;


发生这种重复的原因是由于表设计不周而产生的,增加唯一索引列就可以解决此问题。

 

2、此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

Mysql

alter table tableName add autoID int auto_increment not null;

create table tmp select min(autoID) as autoID from tableName group by Name,Address;

create table tmp2 select tableName.* from tableName,tmp where tableName.autoID = tmp.autoID;

drop table tableName;

rename table tmp2 to tableName;


SQL Server

select identity(int,1,1) as autoID, * into #Tmp from tableName;

select min(autoID) as autoID into #Tmp2 from #Tmp group by Name,Address;

drop table tableName;

select * into tableName from #Tmp where autoID in(select autoID from #Tmp2);

drop table #Tmp;

drop table #Tmp2;

Oracle

DELETE FROM tableName t1 WHERE t1.ROWID > (SELECT MIN(t2.ROWID) FROM tableName t2 WHERE t2.Name = t1.Name and t2.Address = t1.Address);

说明:

1. MySQL和SQL Server中最后一个select得到了Name,Address不重复的结果集(多了一个autoID字段,在大家实际写时可以写在select子句中省去此列)

2. 因为MySQL和SQL Server没有提供rowid机制,所以需要通过一个autoID列来实现行的唯一性,而利用Oracle的rowid处理就方便多了。而且使用ROWID是最高效的删除重复记录方法。

DISCUZ Username 重复:

SELECT max(uid), username, count( * ) AS gg
FROM `cdb_members`
GROUP BY username
ORDER BY gg DESC
LIMIT 0 , 30
posted on 2008-08-09 03:33 Prog 阅读(217) 评论(0)  编辑 收藏 引用 网摘 所属分类: Mysql

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