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:
<div class="codetitle"><a style="CURSOR: pointer" data="45389" class="copybut" id="copybut45389" onclick="doCopy('code45389')"> 代码如下:
<div class="codebody" id="code45389">
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],[Sex] ) Values(2,[Sex] ) Values(3,[Sex] ) Values(4,'Lisa',24,0)
Insert Into Employee ([Id],[Sex] ) Values(5,[Sex] ) Values(6,[Sex] ) Values(7,'M
irsa',23,[Sex] ) Values(8,[Sex] ) Values(9,[Sex] ) Values(10,'John',26,[Sex] ) Values(11,'Abraham',28,[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):
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],[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:
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:
WITH Dups As
(
Select [ID],ROW_NUMBER() OVER(Partition By [ID] Order By (SELECT 0)) AS rn
FROM Employee
)
Select [ID],[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
