SQL Server2008中删除重复记录的方法分享

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SQL Server2008中删除重复记录的方法分享

现在让我们来看在SQL SERVER 2008中如何删除这些记录, 首先,可以模拟造一些简单重复记录:

复制代码代码如下:

	
	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)

OK,首先我们使用最常见的方法:

Delete From Employee Where Name in (select NameFrom Employee Group By Name Having Count(Name)>1);
接着使用RowNumber():

Delete T From( Select Row_Number() Over(Partition By [Name] Order By (SELECT 0)) As RowNumber,* From Employee) TWhere T.RowNumber > 1;

还可以使用CTE (Common Table Expressions):

复制代码代码如下:

	
	With Dups as

	(

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

	FROM Employee

	)

	Delete From Dups

	Where rn>1;

再加上RANK()的CTE:

复制代码代码如下:

	
	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;

下面是这四个T-SQL查询的执行计划:

ExecutionPlan1

你可以看到没有用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。
注意上面的方法只是在重复记录比较少的情况下, 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:

复制代码代码如下:

	
	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'

希望这篇POST对您开发有帮助.作者：Petter Liu

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