Ex6 homework tut 2

Trong TH moi quan he 1-nhieu: lay PK cua bang mot chuyen sang bang nhieu(goi
la FK: foreign key)
Trong TH moi quan he mot-mot: lay PK chuyen sang bangg nao cung duoc, nhung
chi can mot bang, khong can thiet dat ca 2 - chuyen sang weak entity, vi du, giua
student voi course thi lay PK course chuyen sang student.