SQL关联查询详解-优快云博客

本文链接：https://blog.youkuaiyun.com/xiaogaokun/article/details/83348521

本文深入探讨了SQL中关联查询的基本语法，包括内连接、子查询、IN与EXISTS的使用，以及如何查找两个表中相同和不同的数据。通过具体实例，帮助读者掌握不同场景下的SQL查询技巧。

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数据库操作比较欠缺：

本篇为基本语法记录：有两个表，表一（T1）和表二（T2）

关联查出表一有的数据，表二也有的数据

首先：

SELECT A.A FROM [testSql].[dbo].[T2] as A , [testSql].[dbo].[T1] as B where A.A = B.A

那么这个和这个是否是相等呢？

SELECT A.[A] FROM [testSql].[dbo].[T2] as A inner join [testSql].[dbo].[T1] as B on a.[A] = B.[A]

当然，自己查询了 in 和exists 语法，如下操作也是成功了。

SELECT  A.[A] FROM [testSql].[dbo].[T1] as A where A.[A] in ( select B.[A] FROM [testSql].[dbo].[T2] as B)

SELECT A.[A] FROM [testSql].[dbo].[T1] as A where exists(select B.[A] from [testSql].[dbo].[T2] as B where A.[A] = B.[A])

当然还有查找表一存在表二不存在的：

SELECT A.[A] FROM [testSql].[dbo].[T1] as A where A.A  not in(select B.[A] from [testSql].[dbo].[T2] as B where A.[A] = B.[A])

SELECT A.[A] FROM [testSql].[dbo].[T1] as A where not exists(select B.[A] from [testSql].[dbo].[T2] as B where A.[A] = B.[A])

合并双表的：

SELECT A.[A] FROM [testSql].[dbo].[T1] as A union (SELECT B.[A] FROM [testSql].[dbo].[T2] as B)

多表查找