OBJECT-BASED DATA MODEL: PRINCIPLES AND COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 174

asterisk (*) indicates that many instances of the object participate in the relation-
ship. Note the cardinalities between the objects PROVIDER and PROVIDER
INVOICE. One instance of the object PROVIDER may be related to many
instances of the object PROVIDER INVOICE.
Generalization/Specialization Note the superset SERVICE and the subsets
PROCEDURE and TEST. SERVICE is a generalization of PROCEDURE. On the
other hand, TEST is a special type of SERVICE.
BUSINESS OBJECTS
In an object-based data model, as the name itself implies, the business object is
the primary building block. The preliminary step for creating a data model rests
on the identification of the business objects for a given set of information
requirements. After the identification of business objects, the rest of data modeling
follows.
You need to clearly understand the concept of a business object and the methods
for identifying business objects and to explore how the objects relate to one another.
You need to differentiate between an object set and an instance of an object set.
You need to examine the nature of objects and see what different types of objects
exist.
We have been discussing characteristics of objects and named these “attributes.”
What are these attributes for an object? Consider the object EMPLOYEE. An
employee has a definite street address and is attached to a department. Are both
address and department attributes of the object EMPLOYEE? If not, why not? You
need to understand how characteristics qualify to become attributes of objects in a
data model.
Object Sets and Instances
Sets Pursue the example of the object EMPLOYEE. As you know, a rectangular
box indicates the object EMPLOYEE in a data model with the name of the object
written inside the box. It is worthwhile repeating what we mentioned above about
what the box actually represents. It does not just represent a single employee. It
indicates the entire set of employees in the organization. The importance of the
concept of an object set becomes clear when we study the concept of relationships.
Thus the object set EMPLOYEE represented by a box is the collection of all
individual employees. Within the box, imagine the single points as denoting indi-
vidual employees. The box contains a set of such points.
Instances Each point represents an instance or individual occurrence of the
object EMPLOYEE. If you examine a single point in the box, that point would
denote a particular employee. If there are 500 employees in the organization, then
500 individual points make up the box that represents the object EMPLOYEE.
Each of the 500 points represents a specific employee.