SQL 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:
| 代码如下 | 复制代码 |
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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)
Select * From dbo.Employee |
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首先我们使用最常见的方法:
| 代码如下 | 复制代码 |
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Delete From Employee Where Name in ( select Name From Employee Group By Name Having Count(Name)>1); |
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接着使用RowNumber():
| 代码如下 | 复制代码 |
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Delete T From( Select Row_Number() Over(Partition By [Name] Order By (SELECT 0)) As RowNumber,* From Employee) T Where T.RowNumber > 1; With Dups as ( select ROW_NUMBER() Over(Partition by [Name] Order by (SELECT 0)) as rn FROM Employee ) Delete From Dups Where rn>1; 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; |
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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:
| 代码如下 | 复制代码 |
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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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