SQL Server2008中删除重复记录

在Database中可能由于某种原因如用户输入,导入数据失败等导致了重复记录.如果你没有用主键,约束,或来其它机制实现数据完整性,那最后总是重复记录在你的数据库中.现在让我们来看在SQLSERVER2008中如何删除这些记录,首先,可以模拟造一些简单重复记录:

CreateTabledbo.Employee

([Id]intPrimaryKEY,

[Name]varchar(50),

[Age]int,

[Sex]bitdefault1)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(1,'James',25,default)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(2,'James',25,default)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(3,'James',25,default)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(4,'Lisa',24,0)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(5,'Lisa',24,0)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(6,'Lisa',24,0)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(7,'Mirsa',23,0)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(8,'Mirsa',23,0)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(9,'Mirsa',23,0)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(10,'John',26,default)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(11,'Abraham',28,default)

InsertIntoEmployee([Id],[Name],[Age],[Sex])Values(12,'Lincoln',30,default)

Select*Fromdbo.Employee

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OK,首先我们使用最常见的方法:

DeleteFromEmployeeWhereNamein(

selectName

FromEmployeeGroupByNameHavingCount(Name)>1);

接着使用RowNumber():[/code]

DeleteTFrom(

SelectRow_Number()Over(PartitionBy[Name]OrderBy(SELECT0))AsRowNumber,*FromEmployee)T

WhereT.RowNumber>1;

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还可以使用CTE(CommonTableExpressions):

WithDupsas

(

selectROW_NUMBER()Over(Partitionby[Name]Orderby(SELECT0))asrn

FROMEmployee

)

DeleteFromDups

Wherern>1;

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再加上RANK()的CTE:

WITHDupsAs

(

Select[ID],[Name],[Age],[Sex]

,ROW_NUMBER()OVER(PartitionBy[Name]OrderBy(SELECT0))ASrn

,RANK()OVER(PartitionBy[Name]OrderBy(SELECT0))ASrnk

FROMEmployee

)

DELETEFROMDups

WHERErn<>rnk;

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下面是这四个T-SQL查询的执行计划:





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


TableSpool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NestedLoops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。

WITHDupsAs

(

Select[ID],[Name],[Age],[Sex]

,ROW_NUMBER()OVER(PartitionBy[ID]OrderBy(SELECT0))ASrn

FROMEmployee

)

Select[ID],[Name],[Age],[Sex]

INTOdbo.EmployeeDupsTmp

FROMDups

WHERErn=1

DROPTABLEdbo.Employee;

EXECsp_rename'dbo.EmployeeDupsTmp','Employee'