OBJECT-BASED DATA MODEL: PRINCIPLES AND COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 168

Cardinality. Consider the object instances “Henry Porter” and “Tuesday DB class.”
Henry Porter must have enrolled for other classes such as Systems Analysis and
Design, Data Structures, and so on. That means that one instance of the object
STUDENT is related to many instances of the object CLASS. Similarly, on the other
side, one instance of “Tuesday DB class” of the object CLASS is related to many
instances of the object STUDENT. In other words, many students are enrolled for
the “Tuesday DB class.” The indication of numbers of instances of one object relat-
ing to number of instances of another object is known as the cardinality of the rela-
tionship. Cardinality of a relationship is shown as numbers near the lines of the
diamond symbol for the relationship. The variables “m, n” indicate a many-to-many
relationship between two object sets.

Generalization/Specialization. Think about the business object STUDENT. The
object set representing STUDENT includes all students in the university. Both grad-
uate students and undergraduate students are part of the STUDENT object set. The
set of graduate students is one part of the STUDENT object set; the set of under-
graduate students is the other part. Both graduate students and undergraduate
students share most of the attributes of the object called STUDENT. However,
graduate students may have attributes such as undergraduate degree and under-
graduate major that do not apply to undergraduate students. What you observe is
that the set of graduate students is a subset of the superset STUDENT. GRADU-
ATE and UNDERGRADUATE are subsets; STUDENT is the superset. Subsets
and supersets commonly occur in real-world situations. Look at supersets and
subsets from another angle. A subset is a specialization of a superset; the superset
is a generalization of the subsets. As shown in Figure 6-2, a special mark like a “U”
is placed on the line linking the subset to the superset to indicate that the superset
contains the instances of the subset.

Mapping of Real-World Information

We have mentioned that the object-based data model is an intuitive model, a true
model, and a model that represents all types and variations of real-world informa-
tion. With the aid of just the defined set of components and their notations, you are
able to represent any collection of information requirements.
Let us take specific examples of business processes in real-world situations of dif-
ferent types of organizations. Note the following details indicating how you can rep-
resent information requirements with the principles of object-based data modeling.
At this introductory stage, let us describe how portions of the data model diagram
would be completed without getting into any complex issues. We just want to map
the elements of real-world information to the components and notations of the data
model.

Airline Reservation

Business Objects Show object boxes for PASSENGER, AIRLINE, and

FLIGHT.
Attributes

Show attribute ellipses for characteristics of the above three objects.