create table classroom
    (building        varchar(15),
     room_number        varchar(7),
     capacity        numeric(4,0),
     primary key (building, room_number)
    );

create table department
    (dept_name        varchar(20),
     building        varchar(15),
     budget                numeric(12,2) check (budget > 0),
     primary key (dept_name)
    );

create table course
    (course_id        varchar(8),
     title            varchar(50),
     dept_name        varchar(20),
     credits        numeric(2,0) check (credits > 0),
     primary key (course_id),
     foreign key (dept_name) references department
        on delete set null
    );

create table instructor
    (ID            varchar(5),
     name            varchar(20) not null,
     dept_name        varchar(20),
     salary            numeric(8,2) check (salary > 29000),
     primary key (ID),
     foreign key (dept_name) references department
        on delete set null
    );

create table section
    (course_id        varchar(8),
         sec_id            varchar(8),
     semester        varchar(6)
        check (semester in ('Fall', 'Winter', 'Spring', 'Summer')),
     year            numeric(4,0) check (year > 1701 and year < 2100),
     building        varchar(15),
     room_number        varchar(7),
     time_slot_id        varchar(4),
     primary key (course_id, sec_id, semester, year),
     foreign key (course_id) references course
        on delete cascade,
     foreign key (building, room_number) references classroom
        on delete set null
    );

create table teaches
    (ID            varchar(5),
     course_id        varchar(8),
     sec_id            varchar(8),
     semester        varchar(6),
     year            numeric(4,0),
     primary key (ID, course_id, sec_id, semester, year),
     foreign key (course_id,sec_id, semester, year) references section
        on delete cascade,
     foreign key (ID) references instructor
        on delete cascade
    );

create table student
    (ID            varchar(5),
     name            varchar(20) not null,
     dept_name        varchar(20),
     tot_cred        numeric(3,0) check (tot_cred >= 0),
     primary key (ID),
     foreign key (dept_name) references department
        on delete set null
    );

create table takes
    (ID            varchar(5),
     course_id        varchar(8),
     sec_id            varchar(8),
     semester        varchar(6),
     year            numeric(4,0),
     grade                varchar(2),
     primary key (ID, course_id, sec_id, semester, year),
     foreign key (course_id,sec_id, semester, year) references section
        on delete cascade,
     foreign key (ID) references student
        on delete cascade
    );

create table advisor
    (s_ID            varchar(5),
     i_ID            varchar(5),
     primary key (s_ID),
     foreign key (i_ID) references instructor (ID)
        on delete set null,
     foreign key (s_ID) references student (ID)
        on delete cascade
    );

create table time_slot
    (time_slot_id        varchar(4),
     day            varchar(1),
     start_hr        numeric(2) check (start_hr >= 0 and start_hr < 24),
     start_min        numeric(2) check (start_min >= 0 and start_min < 60),
     end_hr            numeric(2) check (end_hr >= 0 and end_hr < 24),
     end_min        numeric(2) check (end_min >= 0 and end_min < 60),
     primary key (time_slot_id, day, start_hr, start_min)
    );

create table prereq
    (course_id        varchar(8),
     prereq_id        varchar(8),
     primary key (course_id, prereq_id),
     foreign key (course_id) references course
        on delete cascade,
     foreign key (prereq_id) references course
    );