drop table student;
drop table enroll;
drop table course;

create table student (
  StuID number,
  StuName varchar2(15),
  Year varchar2(2),
  Age number,
  primary key (stuID),
  check (Year IN ('FR', 'SO', 'JR', 'SR')),
  check (Grade >= 0)
);

create table course (
  Subject varchar2(10),
  Num number,
  Title varchar2(15),
  Fee number,
  primary key (Subject, Num),
  check (Fee >= 0)
);

create table enroll (
  StuID number,
  Subject varchar2(10),
  Num number,
  Grade varchar2(1),
  primary key (StuID, Subject, Num),
  foreign key(StuID) references student on delete cascade,
  foreign key(Subject, Num) references course on delete cascade,
  check (Grade IN ('A', 'B', 'C', 'D', 'F'))
);