如何在SQL Server2008中删除重复记录

如何在SQL SERVER 2008中如何删除重复记录的方法。

首先,制造一些简单重复记录。

复制代码代码示例:

	Create Table dbo.Employee ( 

	[Id] int Primary KEY , 

	[Name] varchar(50), 

	[Age] int, 

	[Sex] bit default 1 

	) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(1,'James',25,default) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(2,'James',25,default) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(3,'James',25,default) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(4,'Lisa',24,0) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(5,'Lisa',24,0) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(6,'Lisa',24,0) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(7,'Mirsa',23,0) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(8,'Mirsa',23,0) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(9,'Mirsa',23,0) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(10,'John',26,default) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(11,'Abraham',28,default) 

	Insert Into Employee ([Id] , [Name] , [Age] , [Sex] ) Values(12,'Lincoln',30,default)

最常见的方法:
Delete From Employee Where Name in (select NameFrom Employee Group By Name Having Count(Name)>1);
使用RowNumber():
Delete T From( Select Row_Number() Over(Partition By [Name] Order By (SELECT 0)) As RowNumber,* From Employee) TWhere T.RowNumber > 1;

使用CTE (Common Table Expressions):

复制代码代码示例:

	With Dups as 

	( 

	select ROW_NUMBER() Over(Partition by [Name] Order by (SELECT 0)) as rn 

	FROM Employee 

	) 

	Delete From Dups 

	Where rn>1;

加上RANK()的CTE:

复制代码代码示例:

	WITH Dups As 

	( 

	Select [ID],[Name],[Age],[Sex] 

	, ROW_NUMBER() OVER(Partition By [Name] Order By (SELECT 0)) AS rn 

	,RANK() OVER(Partition By [Name] Order By (SELECT 0)) AS rnk 

	FROM Employee 

	) 

	DELETE FROM Dups 

	WHERE rn<>rnk;

可以看到没有用CTE的方法开销最大, 主要是在Table Spool, 这里开销了44%, Table Spool 是一个物理运算符。

Table Spool 运算符扫描输入，并将各行的一个副本放入隐藏的假脱机表中，此表存储在 tempdb 数据库中并且仅在查询的生存期内存在。如果重绕该运算符（例如通过 Nested Loops 运算符重绕），但不需要任何重新绑定，则将使用假脱机数据，而不用重新扫描输入。
注意上面的方法只适用于重复记录比较少的情况, 如果重复记录多. DELETE将会非常慢, 最好的方法是复制目标数据到另一个新表,删除原来的表,重命名新表为原来的表. 或用临时表, 这样还可以减少数据库事务日志. 看下面的T-SQL:

复制代码代码示例:

	WITH Dups As 

	( 

	Select [ID],[Name],[Age],[Sex] 

	, ROW_NUMBER() OVER(Partition By [ID] Order By (SELECT 0)) AS rn 

	FROM Employee 

	) 

	Select [ID],[Name],[Age],[Sex] 

	INTO dbo.EmployeeDupsTmp 

	FROM Dups 

	WHERE rn=1 

	DROP TABLE dbo.Employee; 

	EXEC sp_rename 'dbo.EmployeeDupsTmp','Employee'

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如何在SQL Server2008中删除重复记录