[database]

[EER diagram]

Below is a solution(not official solution) schema to the following EER diagram. What I am confused about is the pid attribute in Product being unique. is there a rule/ref behind pid's uniqueness(see below), please explain. The first is the EER diagram in question and then a solution that I am referring to.

Customer(cid, sign date, SSN) PK: cid, unique

Product(pid, price) PK: pid, unique

Movie(pid, year, director, title) FK: pid, references Product, unique

Book(ISBN, title, page number, SSN) PK: ISBN, FK: SSN references author, not null

Person(SSN, first, last, cid) PK: SSN FK: cid, references customer, unique

Business(ID, name, cid) PK: ID, name FK: cid, references customer, unique, not null

Hardcover(ISBN) FK: ISBN references Book, unique, not null

Audiobook(ISBN) FK: ISBN references Book, unique, not null

Paperback(ISBN) FK: ISBN references Book, unique, not null

Author customer(SSN, discount) PK: SSN references author, unique, cid references customer, unique

Author(SSN, start date) PK: SSN, FK: ISBN, references book, unique

Buy(cid, pid) FK: cid references customer unique, pid references product, unique

cid sign date pid price N M Customer buy Product (discount Business title d ISBN title page number director Author Customer Person year ID name Movie Book SSN name N last first write 1 Hardcover Audiobook Paperback Author start date