JS和CSS的多浏览器兼容(3)

浏览数：23 / 时间：2015年06月09日

 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:

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

OK,首先我们使用最常见的方法:

Delete From Employee Where Name in (
select Name
From 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) T
Where T.RowNumber > 1;

 

还可以使用CTE (Common Table Expressions):

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:

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;

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:

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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