–链接查询
–内连接
–inner join…on
–查询有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;
–按照要求显示姓名，班级
select student.*,classes.name from student inner join classes on student.cls_id=classes,id;
–给数据表起名字
select s.*,c.name from student as s inner join classes as c on s.cls_id=c.id;
–在以上查询中，将班级姓名显示在第一列
select c.name，s.* from student as s inner join classes as c on s.cls_id=c.id;
–查询有能够对应班级学生以及班级信息，按照班级进行排序
select c.name，s.* from student as s inner join classes as c on s.cls_id=c.id order by c.name;
–当同一个班级的时候，按照序号排序
select c.name，s.* from student as s inner join classes as c on s.cls_id=c.id order by c.name，s.id;
–左链接
–left join…on
–查询每个学生对应的班级信息
–select * from student as s left join classes as c on s.cls_id=c.id;
–查询没有对应班级信息的学生
—-select * from student as s left join classes as c on s.cls_id=c.id having c,id is null;