OBJECT-BASED DATA MODEL: PRINCIPLES AND COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 192

A B

A is a generalization of B.
B is a specialization of A.
Superset

Subsets

EMPLOYEE HOURLY EMPLOYEE

SALARIED EMPLOYEE

PASSENGER BUSINESS CLASS

ECONOMY CLASS

PATIENT INPATIENT

OUTPATIENT

SHAREHOLDER INDIVIDUAL

INSTITUTIONAL

INSURANCE AUTO

LIFE

Figure 6-17 Supersets and subsets: examples.

POLICY

1,1 1,1

1,1

1,1 1,1

1,1

HEALTH
POLICY

AUTO
POLICY

LIFE
POLICY

1,1 1,1

1,1

1,1 1,1

1,1

VAN
POLICY

CAR
POLICY

TRUCK
POLICY

Figure 6-18 POLICY: generalization/specialization hierarchy levels.

than two levels in the generalization/specialization hierarchy. Figure 6-18 shows
three levels in the hierarchy for POLICY, showing levels of subsets for insurance
policies.
What about the instances in the superset and each of the subsets? The set of
instances within the supertype object is a collection of all the instances in the lower-
level subtype objects. If a particular instance is present in subtype AUTOPOLICY,
then that instance also exists in the supertype POLICY. Note the cardinality indi-
cator “1,1” between the supertype and each subtype object.