Generalization, Specialization and Aggregation | Database Management System

Generalization, Specialization and Aggregation | Database Management System

Generalization, Specialization and Aggregation: Generalization and specialization are both abstraction mechanism used to hide Superfluous details of a set of objects.

For example, when you say Maruti 800, then it is presumed that it is a car and you don’t specify vehicle type such as car, bus or truck.

Generalization

  1. “Generalization is the abstracting process of viewing set of objects as a single general class by concentrating on characteristics of lower level entities while ignoring there is different”.
  2. Generalization is a relationship that exist b/w a high level entity set and one or more lower-level entity Set.

For example person is a high-level entity set and customer and employee are lower-level entity Sets. Higher and lower level entity sets also may be designated by the terms Super-class and sub-class respectively. The person entity is the Super-class of the customer and employee sub-class.

Generalization is represented by the triangle component labelled IS-A. The IS-A stands for “is a” and represents, for example, that customer “is a person. The IS-A relationship may also be referred to as a Super-class sub-class relationship.

Specialization

it is the abstracting process of introducing new characteristics to an existing class of objects to Create on or more new classes of objects. For example, the specialization of a person allow us to distinguish among persons according to whether they are employee or customers. Similar s student can be a regular or part-time student.

Attribute inheritance

A Crucial property of the higher and lower-level entities Created by Specialization and generalization is attribute inheritance. The attributes of higher level entity set are said to be inherited by the lower level entity sets.

For example: Customer and employee inherit the attributes of persons. If an entity Set is a lower-level entity set in more than one ISA relationship, then the entity set has

multiple inheritance, and the resulting structure is said to be a “lattice”. Here Customer and employee are sub-classes, person is the super class.

For example, of generalization:

Fig. ER Diagram with Generalization and Specialization

 

Aggregation

  1. Aggregation is a technique to express relationship among relationship. Through E-R modeling we cannot express relationship among relationships. Thus, we use the concept of aggregation for this concept of aggregation for this concept.
  2. Aggregation is an abstraction through which relationships are treated as entities.

 

Extended ER model

The Extended Entity Relationship Model (EER) include all the modeling concepts of the model and the Concepts of Specialization generalization, higher-level and lower-level entity set attributes inheritance and aggregation.