建立两个模拟表student表和course表

student和course表.jpg

1.cross join

A表和B表求乘积（即笛卡尔积）

select * from student ,course ;
等于
select * from student  CROSS JOIN  course ;

查询的是笛卡尔乘积student（5）Xcourse(4)=20条

select * from student  cross join  course on  student.id = course.student_id;
相同
select * from student , course where  student.id = course.student_id;

查询结果是：

cross join on 结果集.jpg

所有限制条件where是通用的
而on只能运用于配合各种join查询

2. inner join

在没有限制条件的情况下相同

select * from student  inner join course ;
等于
select * from student ,course ;
等于
select * from student  CROSS JOIN  course ;

都是取得20条记录

限制条件下与cross基本相同。
即

select * from student  cross join  course on  student.id = course.student_id;
相同
select * from student , course where  student.id = course.student_id;
相同
select * from student inner join course on  student.id = course.student_id;

3. left join

以左表为主，右表不符合条件的补null

select * from student left join course on  student.id = course.student_id;

结果

left join.jpg

4 left join

以右表为主，左表不符合条件的补null

select * from student right join course on  student.id = course.student_id;

结果：

right join.jpg

左连接可以转成右连接

select * from student left join course on  student.id = course.student_id;
相同
select * from  course right join student on  student.id = course.student_id;

5 on和where

on只能和各种连接联合使用select中
where可以用在各种语句上select 、insert、update、delete .....

注意

select * from student left join course  on 1=1 where  student.id = course.student_id;
相同
select * from student , course  where  student.id = course.student_id;

加上where条件后则不是以左表为主了！