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;

 

转载于:https://www.cnblogs.com/Springmoon-venn/p/5459791.html