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  • SQL之left join、right join、inner join

    创建表(Create table):

                                   

    CREATE TABLE A ( Id INT PRIMARY KEY, Name VARCHAR(20) NOT NULL);

    CREATE TABLE B ( Id INT PRIMARY KEY, Name VARCHAR(20) NOT NULL);

                    插入数据(Insert data):

                                   

    insert into A(id,name) values(1,'Mary1');

    insert into A(id,name) values(2,'Mary2');

    insert into A(id,name) values(3,'Mary3');

    insert into B(id,name) values(1,'Tom1');

    insert into B(id,name) values(2,'Tom2');

    insert into B(id,name) values(4,'Tom4');

     

                    查询a表(Select from a):

                                   

    Select * from a;

                                   

                                   

                    查询b表(Select from b):

                                   

    Select * from b;

                                   

                                   

    内连接(Inner join,自然连接): 内连接只有两个表数据完全匹配才显示查询结果(个人觉得,和不用没什么区别)

                   

    Select a.*,b.* from a inner join b where a.id = b.id;

    Select a.*, b.* from a, b where a.id = b.id;

     

                                

                    左连接(Left join): 显示左表所有数据,只有右表数据和左表匹配是才显示右表数据,如果左表数据在右表中没有匹配的行,用null显示(如下,id = 3)

                                   

    Select a.*,b.* from a left join b on a.id = b.id;

     

    右连接(Right join): 显示右表所有数据,只有左表数据和右表匹配时,才显示左表数据,如果右表数据在左表中没有匹配的行,用null显示(如下,id=4),跟左连接相反

                   

    Select a.*,b.* from a right join b on a.id=b.id;

     

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  • 原文地址:https://www.cnblogs.com/Springmoon-venn/p/5459791.html
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