Strong and Weak Entity Set

1. What is an Entity Set?

An entity set is a collection of similar entities that share the same attributes. Each entity represents a real-world object.

• Student
• Employee
• Department

2. Strong Entity Set

A strong entity set is an entity that has a primary key and can be uniquely identified on its own.

• Has its own primary key
• Independent of other entities
• Represented by a single rectangle

[ STUDENT ]
   |
   |-- Roll_No (Primary Key)
   |-- Name
   |-- Course

3. Weak Entity Set

A weak entity set does not have a primary key of its own. It depends on a strong entity for identification.

• No primary key
• Dependent on strong entity
• Uses partial key (discriminator)

[ DEPENDENT ]
   |
   |-- Dep_Name (Partial Key)
   |-- Age

4. Identifying Relationship

A weak entity is connected to its strong entity using an identifying relationship. This relationship is mandatory.

[ EMPLOYEE ] ==has== [ DEPENDENT ]
     |                    |
  Emp_ID              Dep_Name
 (PK)               (Partial Key)

5. Representation in ER Diagram

• Strong Entity → Single rectangle
• Weak Entity → Double rectangle
• Identifying relationship → Double diamond

+-----------+      ==Has==      +=============+
| EMPLOYEE  |==================|  DEPENDENT   |
+-----------+                  +=============+

6. Example of Strong & Weak Entity

An Employee can exist independently, but a Dependent cannot exist without an employee.

EMPLOYEE (Emp_ID, Name)
DEPENDENT (Dep_Name, Age, Emp_ID)

7. Difference between Strong and Weak Entity Set

Strong Entity                 Weak Entity
---------------------------  ---------------------------
Has primary key              No primary key
Independent existence        Depends on strong entity
Single rectangle             Double rectangle
No identifying relationship  Uses identifying relationship

8. Importance of Strong & Weak Entity Set

• Helps in accurate database modeling
• Ensures correct relationships
• Reduces data redundancy
• Improves database integrity

Practice Questions

1. Define strong entity set.
2. Define weak entity set.
3. What is identifying relationship?
4. Differentiate between strong and weak entity set.
5. Give one real-life example of weak entity.

Practice Task