代码如下:
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)
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):
(本文来源于图老师网站,更多请访问https://m.tulaoshi.com/bianchengyuyan/)代码如下: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。
(本文来源于图老师网站,更多请访问https://m.tulaoshi.com/bianchengyuyan/)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。希望这篇POST对您开发有帮助.作者:Petter Liu