Constraints and Characteristics of Specialization and Generalization Hierarchies




In general, we may have several specializations defined on the same entity type (or superclass), as shown in Figure 8.1. In such a case, entities may belong to subclasses in each of the specializations. However, a specialization may also consist of a single subclass only, such as the {MANAGER} specialization in Figure 8.1; in such a case, we do not use the circle notation. In some specializations we can determine exactly the entities that will become members of each subclass by placing a condition on the value of some attribute of the superclass. Such subclasses are called predicate-defined (or condition-defined) subclasses. For example, if the EMPLOYEE entity type has an attribute Job_type, as shown in Figure 8.4, we can specify the condition of membership in the SECRETARY subclass by the condition (Job_type = ‘Secretary’), which we call the defining predicate of the subclass. This condition is a constraint specifying that exactly those entities of the EMPLOYEE entity type whose attribute value for Job_type is ‘Secretary’ belong to the subclass.We display a predicate-defined subclass by writing the predicate condition next to the line that connects the subclass to the specialization circle. If all subclasses in a specialization have their membership condition on the same attribute of the superclass, the specialization itself is called an attribute-defined specialization, and the attribute is called the defining attribute of the specialization. In this case, all the entities with the same value for the attribute belong to the same subclass. We display an attribute-defined specialization by placing the defining attribute name next to the arc from the circle to the superclass, as shown in Figure 8.4. When we do not have a condition for determining membership in a subclass, the subclass is called user-defined. Membership in such a subclass is determined by the database users when they apply the operation to add an entity to the subclass; hence, membership is specified individually for each entity by the user, not by any condition that may be evaluated automatically.

 

Constraints and Characteristics of Specialization and Generalization Hierarchies

Two other constraints may apply to a specialization. The first is the disjointness (or disjointedness) constraint, which specifies that the subclasses of the specialization must be disjoint. This means that an entity can be a member of at most one of the subclasses of the specialization. A specialization that is attribute-defined implies the disjointness constraint (if the attribute used to define the membership predicate is single-valued). Figure 8.4 illustrates this case, where the d in the circle stands for disjoint. The d notation also applies to user defined subclasses of a specialization that must be disjoint, as illustrated by the specialization {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} in Figure 8.1. If the subclasses are not constrained to be disjoint, their sets of entities may be overlapping; that is, the same (real-world) entity may be a member of more than one subclass of the specialization. This case, which is the default, is displayed by placing an o in the circle, as shown in Figure 8.5.

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The second constraint on specialization is called the completeness (or totalness) constraint, which may be total or partial. A total specialization constraint specifies that every entity in the superclass must be a member of at least one subclass in the specialization. For example, if every EMPLOYEE must be either an HOURLY_EMPLOYEE or a SALARIED_EMPLOYEE, then the specialization {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} in Figure 8.1 is a total specialization of EMPLOYEE. This is shown in EER diagrams by using a double line to connect the superclass to the circle.

A single line is used to display a partial specialization, which allows an entity not to belong to any of the subclasses. For example, if some EMPLOYEE entities do not belong to any of the subclasses {SECRETARY, ENGINEER, TECHNICIAN} in Figures 8.1 and 8.4, then that specialization is partial. Notice that the disjointness and completeness constraints are independent.

Hence,we have the following four possible constraints on specialization:
? Disjoint, total
? Disjoint, partial
? Overlapping, total
? Overlapping, partial

Constraints and Characteristics of Specialization and Generalization Hierarchies

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Of course, the correct constraint is determined from the real-world meaning that applies to each specialization. In general, a superclass that was identified through the generalization process usually is total, because the superclass is derived from the subclasses and hence contains only the entities that are in the subclasses. Certain insertion and deletion rules apply to specialization (and generalization) as a consequence of the constraints specified earlier.

Some of these rules are as follows:
? Deleting an entity from a superclass implies that it is automatically deleted from all the subclasses to which it belongs.
? Inserting an entity in a superclass implies that the entity is mandatorily inserted in all predicate-defined (or attribute-defined) subclasses for which the entity satisfies the defining predicate.
? Inserting an entity in a superclass of a total specialization implies that the entity is mandatorily inserted in at least one of the subclasses of the specialization. 



Frequently Asked Questions

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Ans: Specialization is the process of defining a set of subclasses of an entity type; this entity type is called the superclass of the specialization. We use the term generalization to refer to the process of defining a generalized entity type from the given entity types. view more..
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