inner join …on 内连接 两张表中有共同对应的信息，取交集 查询能够对应班级的学生以及班级信息
select * from student inner join classes on student.cls_id=classes.id;查询显示姓名和班级
select s.name,c.name from student as s inner join classes as c on s.cls_id=c.id;//s.*表示student所有信息查询 能够对应的班级的学生以及班级名称，按照班级进行排序
select s.*,c.name from student as s inner join classes as c where s.cls_id=c.id order by c.name desc;