Generalization

Generalization is the opposite of specialization. It is the process of combining two or more lower-level entities into a higher-level entity (superclass) based on their common attributes.

• It answers the question: “What common entity can these subtypes belong to?”
• It is a bottom-up approach.

Key Points:

1. Bottom-up approach (starts with specific entities and abstracts them into a generalized entity).
2. Used when multiple entities share common attributes.
3. Avoids redundancy by placing shared attributes in the generalized superclass.

Example

Suppose we have two entities:

• Car → attributes: Car_ID, Model, Color
• Bike → attributes: Bike_ID, Model, Color

Both share common attributes: Model, Color. Instead of keeping them separately with redundant attributes, we can create a generalized entity:

• Superclass → Vehicle (Vehicle_ID, Model, Color)
• Subclasses → Car, Bike

ER Representation (textual):

       Car         Bike
   (Car_ID, ...)  (Bike_ID, ...)
        \          /
         \        /
          Vehicle
     (Vehicle_ID, Model, Color)

Here, Generalization merges Car and Bike into a general entity Vehicle.

Difference Between Specialization and Generalization

Aspect	Specialization	Generalization
Approach	Top-down (from general to specific)	Bottom-up (from specific to general)
Purpose	To create subtypes with distinct attributes	To merge common attributes into a single entity
Example	Employee → FullTime, PartTime	Car + Bike → Vehicle
Relationship type	“IS-A” (Employee is a FullTime)	“IS-A” (Car is a Vehicle)