新建两张表:
course: student:
1、外连接:左连接、右连接和完全外连接
左连接:select * from student left join course on student.ID=course.ID
右连接:select * from student right join course on student.ID=course.ID
完全外连接:select * from student full join course on student.ID=course.ID
2、内连接:select * from student inner join course on student.ID=course.ID(select * from student,course where student.ID=course.ID)
3、交叉连接:select * from student cross join course