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Chapter: Fundamentals of Database Systems : Conceptual Modeling and Database Design : The Enhanced Entity-Relationship (EER) Model

Specialization and Generalization

Specialization is the process of defining a set of subclasses of an entity type; this entity type is called the superclass of the specialization.

Specialization and Generalization


1. Specialization


Specialization is the process of defining a set of subclasses of an entity type; this entity type is called the superclass of the specialization. The set of subclasses that forms a specialization is defined on the basis of some distinguishing characteristic of the entities in the superclass. For example, the set of subclasses {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of the superclass EMPLOYEE that distinguishes among employee entities based on the job type of each employee entity. We may have several specializations of the same entity type based on different distinguishing characteristics. For example, another specialization of the EMPLOYEE entity type may yield the set of subclasses {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}; this specialization distinguishes among employees based on the method of pay.


Figure 8.1 shows how we represent a specialization diagrammatically in an EER diagram. The subclasses that define a specialization are attached by lines to a circle that represents the specialization, which is connected in turn to the superclass. The subset symbol on each line connecting a subclass to the circle indicates the direction of the superclass/subclass relationship. Attributes that apply only to entities of a particular subclass—such as TypingSpeed of SECRETARY—are attached to the rec-tangle representing that subclass. These are called specific attributes (or local attributes) of the subclass. Similarly, a subclass can participate in specific relation-ship types, such as the HOURLY_EMPLOYEE subclass participating in the BELONGS_TO relationship in Figure 8.1. We will explain the d symbol in the circles in Figure 8.1 and additional EER diagram notation shortly.


Figure 8.2 shows a few entity instances that belong to subclasses of the {SECRETARY, ENGINEER, TECHNICIAN} specialization. Again, notice that an entity that belongs to a subclass represents the same real-world entity as the entity connected to it in the EMPLOYEE superclass, even though the same entity is shown twice; for example, e1 is shown in both EMPLOYEE and SECRETARY in Figure 8.2. As the figure suggests, a superclass/subclass relationship such as EMPLOYEE/ SECRETARY somewhat resembles a 1:1 relationship at the instance level (see Figure 7.12). The main difference is that in a 1:1 relationship two distinct entities are related, whereas in a superclass/subclass relationship the entity in the subclass is the same real-world entity as the entity in the superclass but is playing a specialized role—for example, an EMPLOYEE specialized in the role of SECRETARY, or an EMPLOYEE specialized in the role of TECHNICIAN.

There are two main reasons for including class/subclass relationships and specializations in a data model. The first is that certain attributes may apply to some but not all entities of the superclass. A subclass is defined in order to group the entities to which these attributes apply. The members of the subclass may still share the majority of their attributes with the other members of the superclass. For example, in Figure 8.1 the SECRETARY subclass has the specific attribute Typing_speed, whereas the ENGINEER subclass has the specific attribute Eng_type, but SECRETARY and ENGINEER share their other inherited attributes from the EMPLOYEE entity type.


The second reason for using subclasses is that some relationship types may be participated in only by entities that are members of the subclass. For example, if only HOURLY_EMPLOYEES can belong to a trade union, we can represent that fact by creating the subclass HOURLY_EMPLOYEE of EMPLOYEE and relating the subclass to an entity type TRADE_UNION via the BELONGS_TO relationship type, as illus-trated in Figure 8.1.


In summary, the specialization process allows us to do the following:


        Define a set of subclasses of an entity type


        Establish additional specific attributes with each subclass


        Establish additional specific relationship types between each subclass and other entity types or other subclasses


2. Generalization


We can think of a reverse process of abstraction in which we suppress the differences among several entity types, identify their common features, and generalize them into a single superclass of which the original entity types are special subclasses. For example, consider the entity types CAR and TRUCK shown in Figure 8.3(a). Because they have several common attributes, they can be generalized into the entity type VEHICLE, as shown in Figure 8.3(b). Both CAR and TRUCK are now subclasses of the generalized superclass VEHICLE. We use the term generalization to refer to the process of defining a generalized entity type from the given entity types.


Notice that the generalization process can be viewed as being functionally the inverse of the specialization process. Hence, in Figure 8.3 we can view {CAR, TRUCK} as a specialization of VEHICLE, rather than viewing VEHICLE as a generalization of CAR and TRUCK. Similarly, in Figure 8.1 we can view EMPLOYEE as a generalization of SECRETARY, TECHNICIAN, and ENGINEER. A diagrammatic notation to distinguish between generalization and specialization is used in some design methodologies. An arrow pointing to the generalized superclass represents a generalization, whereas arrows pointing to the specialized subclasses represent a specialization. We will not use this notation because the decision as to which process is followed in a particular situation is often subjective. Appendix A gives some of the suggested alternative dia-grammatic notations for schema diagrams and class diagrams.


So far we have introduced the concepts of subclasses and superclass/subclass relationships, as well as the specialization and generalization processes. In general, a

superclass or subclass represents a collection of entities of the same type and hence also describes an entity type; that is why superclasses and subclasses are all shown in rectangles in EER diagrams, like entity types. Next, we discuss the properties of specializations and generalizations in more detail.

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