-- 多对多的秘密：项目中，往往把关系分拆出一张单独表

--维表（学生维度）

create table t_stu(

   id number primary key,

   user_name varchar2(10)

);

insert into t_stu values(1,'小军');

insert into t_stu values(2,'小黄');

commit

select * from t_stu

drop table t_ke

--维表（课程维度）

create table t_ke(

   id number primary key,

   ke_name varchar2(100)

);

insert into t_ke values(1,'高等数学');

insert into t_ke values(2,'离散数学');

insert into t_ke values(3,'牛顿法');

insert into t_ke values(4,'大学英语');

insert into t_ke values(5,'马克思主义');

insert into t_ke values(6,'江泽明思想');

insert into t_ke values(7,'邓小平理论');

commit

select * from t_ke

-- 事实表

CREATE TABLE t_stu_ke(

  id NUMBER PRIMARY KEY,

  user_id NUMBER(5),

  ke_id NUMBER(5)

);

delete from t_stu_ke

insert into t_stu_ke values(1,1,1);

insert into t_stu_ke values(2,1,2);

insert into t_stu_ke values(3,1,6);

commit;

insert into t_stu_ke values(4,2,4);

insert into t_stu_ke values(5,2,6);

commit;

 

select sk.id,s.user_name,k.ke_name from t_stu_ke sk

left join t_stu s on sk.user_id =s.id

left join t_ke k on sk.ke_id=k.id

 

insert into t_stu_ke values(6,3,3);

insert into t_stu_ke values(7,2,8);

commit

 

select

   sk.id,

   nvl(s.user_name,'未知学生:'||sk.user_id) 学生,

   nvl(k.ke_name,'未知课程:'||sk.ke_id) 选课

from t_stu_ke sk

   left join t_stu s on sk.user_id =s.id

   left join t_ke k on sk.ke_id=k.id

  

-- 站在不通的维度看世界  

   group by