ENTITY-RELATIONSHIP DATA MODEL - Database Design and Development: An Essential Guide for IT Professionals - page 210

EMPLOYEE
Name, Phone, SocSecNo

employee-1

Linda, 212-324-7798, 111-22-3333

employee-2

Dan, 516-223-8193, 321-54-9876

employee-3

Reisha, 732-421-1234, 445-37-7698

DEPARTMENT
Number, DeptName, Location

department-1

101, Administration, New York

department-2

201, Marketing, Chicago

department-3

301, Finance, Boston

Figure 7-4 Entity sets.

Strong or
regular
entity types

CUSTOMER ORDER

Independent existence Independent existence

Weak
entity type

ORDER
DETAIL

Existence of ORDER DETAIL
depends on ORDER

Figure 7-5 Strong and weak entity types.

pendently. Similarly, each order entity is uniquely identified by an order number
and therefore, can exist without dependence on entities of other entity types.
Contrast this with the case of the entity type ORDER DETAIL. An order detail
is not uniquely identifiable unless it is related to the corresponding order to which
it is an order detail. That is, an order detail entity has no independent existence
apart from an order entity. The ORDER DETAIL entity type depends on the
ORDER entity type for its existence.
ORDER DETAIL is called a weak entity type. CUSTOMER and ORDER are
known as regular or strong entity types. A weak entity type depends on another
regular entity type for its existence. Weak entities depend on the corresponding
strong entities for identification. We will review weak entities further in later
sections.