Database design supertypes and subtypescopyright

Objectives

Purpose

needs to track payments
from customers. CREDIT CARD• Customers can pay by cash,
by check, or by credit card.

Evaluating Entities

• All payments have some common

• And for credit card and check
payments, we may need to know CREDIT CARD

which CUSTOMER made the
payment, while this is not needed for cash payments.

Evaluating Entities

• In this case, the entity is called a

• A subtype:
– Inherits all attributes of the supertype

– Never exists alone

Supertype Example

EXAM SUPERTYPE

• This idea leads to the two subtype rules:
– Exhaustive: Every instance of the supertype is also an instance of one of the subtypes. All subtypes are listed without
omission.

– Mutually Exclusive: Each instance of a supertype is an instance of only one possible subtype.

Always More Than One Subtype

WALLCOVERING SUPERTYPE

(exhaustive)

Terminology

Key terms used in this lesson included:

Summary