Explain enhanced entity relationship model in detail. What is aggregation?

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 EER Model

EER is a high-level data model that incorporates the extensions to the original ER model. It is a diagrammatic technique for displaying the following concepts

  1. Sub Class and Super Class
  2. Specialization and Generalization
  3. Union or Category
  4. Aggregation

These concepts are used when they come in the EER schema and the resulting schema diagrams are called EER Diagrams.

Features of the EER Model

  • EER creates a design more accurate to database schemas.
  • It reflects the data properties and constraints more precisely.
  • It includes all modeling concepts of the ER model.
  • The diagrammatic technique helps in displaying the EER schema.
  • It includes the concept of specialization and generalization.
  • It is used to represent a collection of objects that is the union of objects of different entity types.

A. Sub Class and Super Class

  • Sub-class and Super class relationships lead to the concept of Inheritance.
  • The relationship between sub-class and superclass is denoted by the symbol.

1. Super Class

  • Super class is an entity type that has a relationship with one or more subtypes. An entity cannot exist in a database merely by being a member of any superclass. 
  • For example, the Shape super class is having sub-groups such as Square, Circle, and Triangle.

2. Sub Class

  • A subclass is a group of entities with unique attributes. The subclass inherits properties and attributes from its superclass.
  • For example squares, circles, and triangles are the sub-class of the Shape superclass.

B. Specialization and Generalization

1. Generalization

  • Generalization is the process of generalizing the entities which contain the properties of all the generalized entities.
  • It is a bottom approach, in which two lower-level entities combine to form a higher-level entity.
  • Generalization is the reverse process of Specialization.
  • It defines a general entity type from a set of the specialized entity type.
  • It minimizes the difference between the entities by identifying the common features.
For example:


In the above example, Tiger, Lion, and Elephant can all be generalized as Animals.

2. Specialization

  • Specialization is a process that defines a group of entities that is divided into subgroups based on their characteristic.
  • It is a top-down approach, in which one higher entity can be broken down into two lower-level entities.
  • It maximizes the difference between the members of an entity by identifying the unique characteristic or attributes of each member.
  • It defines one or more subclass for the superclass and also forms the superclass/subclass relationship.



In the above example, Employees can be specialized as Developers or Tester, based on what role they play in an Organization.

C. Category or Union

  • The category represents a single superclass or subclass relationship with more than one superclass.
  • It can be total or partial participation.
  • For example Car booking, the Car owner can be a person, a bank (who holds possession of a Car), or a company. Category (subclass) → Owner is a subset of the union of the three super classes → Company, Bank, and Person. A Category member must exist in at least one of its superclasses.


D. Aggregation

  • Aggregation is a process that represents a relationship between a whole object and its component parts.
  • It abstracts a relationship between objects and views the relationship as an object.
  • It is a process in which two entity is treated as a single entity.
In the above example, the relation between College and Course is acting as an Entity in Relation with Student.

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