有三张表，学生表S，课程表C，学生课程表SC，学生可以选修多门课程，一门课程可能被多个学生选修，通过SC表关联。
（1） 写出建表以及插入语句；
（2） 写出SQL语句，查询选修了所有选修课程的学生；
（3） 写出SQL语句，查询选修了至少2门以上的课程的学生。
ANSWER:
（1） 写出建表以及插入语句；
/

create table student (
id number(10) primary key,
name varchar2(20));

create table course (
id number(10) primary key,
name varchar2(20));

create table sc(
sid number(10) references student(id),
cid number(10) references course(id),
grade number(42));

INSERT VALUES:

2） 写出SQL语句，查询选修了所有选修课程的学生；

select s.id,s.name
from student s
where s.id in(
select sid
from sc
group by sid
having count(*)=
(select count(*)
from course)
);

（3） 写出SQL语句，查询选修了至少2门以上的课程的学生。

select sid,count(*) from sc group by sid having count(*)>=2;

SELECT S.ID,S.NAME
FROM STUDENT S
WHERE S.ID IN(
select sid
from sc
group by sid
having count(*)>=2
)

drop table student cascade constraints;
drop table course cascade constraints;
drop table sc;

commit;

create table student(
id number(10) primary key,
name varchar2(15)
);

create table course(
id number(10) primary key,
name varchar2(15)
);

create table sc(
sid number(10) references student(id),
cid number(10) references course(id)
);

commit;

insert into student values(1,’star’);
insert into student values(2,’moon’);
insert into student values(3,’oracle’);
insert into student values(4,’sun’);

insert into course values(1,’Java’);
insert into course values(2,’C++’);
insert into course values(3,’Chinese’);
insert into course values(4,’English’);

insert into sc values(1,1);
insert into sc values(1,2);
insert into sc values(1,3);
insert into sc values(1,4);
insert into sc values(2,1);
insert into sc values(2,2);
insert into sc values(2,3);
insert into sc values(3,1);
insert into sc values(3,2);
insert into sc values(4,3);
insert into sc values(4,4);
commit;
;