Generalization is a top‑down design technique in the Entity Relationship (ER) Model where several specific entity types are combined into a more general entity type (called a superclass). This approach is used to remove redundancy and group common attributes and relationships at a higher level.
Generalization is the parent → children view of entity types and is often shown using IS‑A relationships (for example, EMPLOYEE IS‑A PERSON).
What is Generalization?
In generalization:
Multiple specialized entity types are merged into a generalized entity type.
The generalized entity type is called the superclass.
The specialized entity types are called subclasses.
For example:
STUDENT and EMPLOYEE may both be generalized into a PERSON entity.
PERSON becomes the superclass; STUDENT and EMPLOYEE become subclasses.
Common attributes such as Name, Address, and Phone can be moved to PERSON, while role‑specific attributes like Roll_No or Salary remain in the subclass.
Why Use Generalization?
To avoid repeating common attributes in multiple entities.
To design a clean, hierarchical structure of entities.
To make the schema easier to extend (for example, adding a new subclass like ALUMNI later).
Example of Generalization
Consider:
STUDENT with attributes: Roll_No, Branch, CGPA
EMPLOYEE with attributes: Emp_ID, Dept, Salary
If both STUDENT and EMPLOYEE share common attributes like Name, Address, Phone, you can:
Create a PERSON superclass with Name, Address, Phone.
Let STUDENT and EMPLOYEE be subclasses of PERSON, each adding its role‑specific attributes.
In this model, every STUDENT IS‑A PERSON, and every EMPLOYEE IS‑A PERSON.
Notation in ER Diagram
The superclass is usually shown as a rectangle labeled as the generalized type (for example, PERSON).
The subclasses are shown as connected rectangles below it.
An IS‑A or generalization arrow points from the subclass to the superclass.
Some notations use hierarchy lines between the superclass and subclasses.
Relationship with Other Concepts
Generalization is the opposite of Specialization (which starts from a general entity and creates more specific ones).
Together, they are used in Enhanced ER (EER) modeling to represent complex entity hierarchies.
Summary
Generalization in the DBMS ER Model is a top‑down technique where specific entity types are grouped into a more general superclass. It helps organize data hierarchically, reduces redundancy, and makes the schema easier to maintain and extend. For beginners, it is a useful way to think about common attributes and relationships across different types of entities.