自关联操作       

 

多表连接查询  inner  join 内查询   left  join  左查询   right  join  右查询 

                                                                                                       

 

select student.id,student.name,classes.name from student inner join classes on student.cls_id = classes.id;                                                                

  inner join内连 left join左连 right join右连     表名join 表名 on 连接字段        

                      

 select student.name,course_student.cid from student left join course_student on student.id = course_student.sid;                                                            

  以student.id与course_student.sid连接匹配,打印student.name,course_student.cid         

                       

    select student.name,course.name from student left join course_student on student.id = course_student.sid left join course on course_student.cid =course.id;                                                                    三表(2明细表+1关系表)相连取交集打印学生名字与课程名字       

                       

select * from areas as p inner join areas as a on p.aid = a.pid where p.atitle = '河南省';                                                                

      省、市，两表查询在一张表里(as起别名)用上级id打印，                                    

 

 select * from areas as p inner join areas as c on c.pid = p.aid inner join areas as a on a.pid = c.aid where a.atitle = '二七区';                                                            

      省、市、区，三表查询在一张表里(as起别名)用上级id打印区表title为‘二七区’的数据     

 

select * from from areas as p inner join areas as c on c.pid = p.aid inner join areas as  as a on a.pid = c.aid where p.atitle = '河南省';     

  打印河南省所有的市和所有的区