这次介绍一下T-SQL中“Not IN” 和“Not Exists”的优化。
Not IN 和 Not Exists 命令 :
有些情况下，需要select/update/delete 操作孤立数据。孤立数据：不存在主表中而存在其关联表中。
操作这样的数据，一般第一反应是利用“Not in” 或 “Not Exists”命令。使用Not IN会严重影响性能，因为这个命令会逐一检查每个记录，就会造成资源紧张，尤其是当对大数据进行更新和删除操作时，可能导致资源被这些操作锁住。
选择NOT IN 还是 NOT Exists
现在SQL Server 中有两个命令可以使用大数据的插入、更新、删除操作，性能方面比NOT IN有很大的提高，语法简单比NOT Exists好很多，写出来的语句看上去很清爽。 现在就请它们闪亮登场，Merge 和 Except。
例子:
首先创建两个表
use [MyTest] create table Test1 ([id] int, [name] varchar(20)) create table Test2 ([id] int, [name] varchar(20), [address] varchar(100))
declare @RowC int declare @Name varchar(20) set @RowC = 0 while @RowC < 400000 Begin set @Name = 'TestName' + CAST(@RowC as varchar(10)) insert into Test1(id, name) values(@RowC, @Name) set @RowC = @RowC+1 end declare @RowC int declare @Name varchar(20) declare @Address varchar(100) set @RowC = 0 while @RowC < 500000 Begin set @Name = 'TestName' + CAST(@RowC as varchar(10)) set @Address = 'TestAddress' + CAST(@RowC as varchar(10)) insert into Test2([id], [name], [address]) values(@RowC, @Name, @Address) set @RowC = @RowC+1 end SELECT [name] FROM Test2 where [name] not in (select [name] from Test1) UPDATE Test2 SET [name] =N'New_Name' where [name] not in (select [name] from Test1) DELETE Test2 FROM Test2 where [name] not in (select [name] from Test1) merge Test2 T using (select name from Test2 except select name from Test1 )S on t.name=s.name when matched then update SET name=N'New_Name' ; merge Test2 T using (select name from Test2 except select name from Test1 )S on t.name=s.name when matched then delete ; SELECT * FROM Test2 S where not exists (select 1 from Test2 inner join Test1 on Test2.name=Test1.name and Test2.name=s.name)
注意，上面还是有一部分使用了Not Exists：
SELECT name FROM Test2 S where not exists (select 1 from Test2 inner join Test1 on Test2.name=Test1.name and Test2.name=s.name)现在需要使用简洁的Except：
select name from Test1 except select name from Test2
转载于:https://www.cnblogs.com/zoro-zero/p/7681402.html
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