Enhanced Entity Relationship Model (EER Model)

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
  • Sub Class and Super Class
  • Specialization and Generalization
  • Union or Category
  • Aggregation
These concepts are used when the comes in EER schema and the resulting schema diagrams called as EER Diagrams.

Features of 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.
  • Diagrammatic technique helps for displaying the EER schema.
  • It includes the concept of specialization and generalization.
  • It is used to represent a collection of objects that is union of objects of different of different entity types.
A. Sub Class and Super Class
  • Sub class and Super class relationship leads the concept of Inheritance.
  • The relationship between sub class and super class is denoted with symbol.
1. Super Class
  • Super class is an entity type that has a relationship with one or more subtypes.
  • An entity cannot exist in database merely by being member of any super class.
    For example: Shape super class is having sub groups as Square, Circle, Triangle.
2. Sub Class
  • Sub class is a group of entities with unique attributes.
  • Sub class inherits properties and attributes from its super class.
    For example: Square, Circle, Triangle are the sub class of Shape super class.
subclass superclass relationship

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 specialized entity type.
  • It minimizes the difference between the entities by identifying the common features.
    For example:
generalization
In the above example, Tiger, Lion, Elephant can all be generalized as Animals.

2. Specialization
  • Specialization is a process that defines a group entities which is divided into sub groups based on their characteristic.
  • It is a top down approach, in which one higher entity can be broken down into two lower level entity.
  • 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 sub class for the super class and also forms the superclass/subclass relationship.
    For example
specialization
In the above example, Employee can be specialized as Developer or Tester, based on what role they play in an Organization.

C. Category or Union
  • Category represents a single super class or sub class relationship with more than one super class.
  • It can be a total or partial participation.
    For example Car booking, Car owner can be a person, a bank (holds a possession on a Car) or a company. Category (sub class) → 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 super classes.
categories union type

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