EER - Enhanced Entity Relationship Model

EER - Enhanced Entity Relationship Model is a high-level data model that incorporates the extensions to the original Entity Relationship model. It is a technique for displaying the concepts such as

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

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 modelling 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 entity types.

1.Sub Class and Super Class:

• Sub class and Super class relationship leads the concept of Inheritance.

Super Class:

• Super class is an entity type that has a relationship with one or more subtypes.
• For example: Shape super class is having sub groups as Square, Circle, Triangle.

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.

2.Specialization and Generalization:

Specialization(Top to Bottom):

•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: In the below example, Employee can be specialized as Developer or Tester, based on what role they play in an Organization.

Generalization(Bottom to Top):

•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: In the below example, Tiger, Lion, Elephant can all be generalized as Animals.

3.Union or Category:

•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 - Person, Bank, Company

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 → Person, Bank, Company. A Category member must exist in at least one of its super classes.

4.Aggregation:

•Aggregation is a process that represents 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.

For example: In the below example, the relation between College and Course is acting as an Entity in Relation with Student.