SQL Server 中左连接和右连接的区别 [英] Difference between left join and right join in SQL Server

问题描述

我了解 SQL Server 中的联接.

I know about joins in SQL Server.

例如.有两个表Table1，Table2.

For example. There are two tables Table1, Table2.

它们的表结构如下.

create table Table1 (id int, Name varchar (10))

create table Table2 (id int, Name varchar (10))

表1数据如下:

    Id     Name     
    -------------
    1      A        
    2      B    

Table2数据如下:

Table2 data as follows:

    Id     Name     
    -------------
    1      A        
    2      B 
    3      C

如果我执行下面提到的两个 SQL 语句，两个输出将是相同的

If I execute both below mentioned SQL statements, both outputs will be the same

select *
from Table1
  left join Table2 on Table1.id = Table2.id

select *
from Table2
  right join Table1 on Table1.id = Table2.id

请说明上述SQL语句中左连接和右连接的区别.

Please explain the difference between left and right join in the above SQL statements.

推荐答案

Select * from Table1 left join Table2 ...

和

Select * from Table2 right join Table1 ...

确实是完全可以互换的.但是尝试Table2 left join Table1(或它的相同对，Table1 right join Table2)以查看差异.这个查询应该给你更多的行，因为 Table2 包含一个 id 在 Table1 中不存在的行.

are indeed completely interchangeable. Try however Table2 left join Table1 (or its identical pair, Table1 right join Table2) to see a difference. This query should give you more rows, since Table2 contains a row with an id which is not present in Table1.

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