drop table class cascade;
create table class (
	gnum integer references grade(num),
	num integer check(num between 1 and 8),
	charge text not null,
	primary key (gnum, num)
);
insert into class values(1, 1, '田中');
insert into class values(1, 2, '佐藤');
insert into class values(1, 3, '鈴木');
insert into class values(1, 4, '松井');
insert into class values(2, 1, '岡田');
insert into class values(2, 2, '山本');
insert into class values(2, 3, '斎藤');
insert into class values(2, 4, '吉田');
insert into class values(3, 1, '村山');
insert into class values(3, 2, '森田');
insert into class values(3, 3, '原田');
insert into class values(3, 4, '島田');