2.3 Self Review Questions
1. Mapping of object-based data model to relational model involves
transformation of some features. List the features before transformation and after transformation.
2. List and describe briefly five attributes of object-based data model.
3. What do you understand about object space diagram? How it differs from
object-based data model?
4. Explain the concept of inheritance. How many types of inheritance available?
Give example for each.
5. What types of objects derived from object classification?
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3 Introduction
Enhanced Entity Relationship Diagram is a conceptual data modeling technique that actually adds some object-oriented concepts in ERD. These additional concepts include
subclassessuperclasses, specializationgeneralization, categories and attribute inheritance.
It is important for students to understand that EERD includes all modeling concepts of basic ERD but it does not attempt to cover all object concepts. In order to develop
EERD it is crucial to know which notations to be adopted. In this practical, Chen notation will be used in all parts. Students will have the opportunity to develop EERD
using UML notations in Chapter 4.
The objectives of this practical include:
To identify the essential objects components added in EERD To define constraints on specialization generalization
To Illustrate specialization generalization constraints in both, EERD and UML
To develop a complete conceptual design with EERD
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3.1 Theoretical Background
3.1.1 Constraints and characteristics of specialization and generalization
In fact, more than one specialization can be defined on the same entity type superclass as shown in Figure 3-1. In this case, entities may belong to each of subclass.
Nevertheless, a specialization may also involve only a single subclass such {Supervisor} subclass in the figure. No circle notation is required for this type of
specialization.
Worker
Clerk Consultant
Physician FirstName
MidName LastName
Name BirthDate
Address WorkCategory
ContractStat CertLevel
PhysClass
d
WorkCategory
Clerk
Consultant Physician
IcNum Supervisor
Administration
Figure 3-1 : Specialization of superclass Worker
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There are a few types of constraints that determine specialization generalization of entities. These include:
a. Constraints based on attribute of superclass
If all subclasses in a specialization are defined based on the same attribute of superclass,
the specialization itself is called attribute-defined specialization. The attribute that involves in the specialization is called defining attribute. In EERD, the defining
attribute name is placed next to the line from the circle to the superclass. Figure 3-1 shows an example of a defining attribute called WorkCategory.
b. Constraints based on value of attribute of superclass
In other case, specialization may also be based on the value of some attribute of the superclass. In other words, the entities that will become the member of each superclass
are determined by placing a condition on the value of attributes of the superclass. The
subclasses generated as the result of this type of specialization is called predicate- defined or conditioned-defined subclasses.
The value of attribute which involves in the specialization is called defining predicate.
In EERD, the defining predicate name is placed next to the line that connects the subclass to the specialization circle.
In Figure 3-1, the values of WorkCategory attribute of superclass Worker are used to determine subclasses. For instance, membership in the CONSULTANT subclass is
determined by the predicate WorkCategory = ‘Consultant’. Therefore, all entities in the WORKER entity type whose attribute value for WorkCategory is ‘Consultant’ must
belong to subclass CONSULTANT.
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c. Constraints based on user
Kategori