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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