Entity in ER Model

Let’s explain each type in detail

1. Strong Entity

A Strong Entity is an entity that:

• Exists independently in the database. It does not rely on any other entity for its existence.
• Always has a Primary Key that uniquely identifies each record (row) in its table. The Primary Key is the main attribute (or set of attributes) used to distinguish one record from another.

Example: Student Table

In the following table each student entity is the strong entity because it can be uniquely identified by Student_ID.

• A single student with → (Student_ID = 101, Name = Ali, Age = 20, Dept = CS)

Strong Entity Notation in ER Model:

In an ER (Entity-Relationship) Model, a Strong Entity is represented by a single rectangle.

In the above diagram, the student is an entity, and Student_ID, Name, Age, and Dept are the attributes.

2. Weak Entity

• Refers to a single real-world object (record) that cannot be uniquely identified by its own attributes alone.
• It is dependent on a Strong Entity for its identification.
• Identified by Partial Key + Primary Key of Strong Entity.

Example: Student and Exam_Result Table

• Problem: Dependent_Name is not unique (many employees can have a dependent named “Ali”).

Solution: How strong entity and weak entity work together is given below

• Emp_ID (from Employee = Strong Entity) + Subject_Name (partial key), together, uniquely identify the weak entity.

Weak Entity Notation in ER Model:

In an ER (Entity-Relationship) Model, a Weak Entity is represented by a double rectangle. In the following diagram, Exam_Result is the weak entity.

3. Entity Set

An Entity Set is simply a collection of similar entities that share the same attributes in a database.

• Each entity is one instance (or record) in the set.
• An entity set is like a table in a relational database.
• The attributes of the entity set define what information is stored for each entity.

Example: Student Entity Set (Strong entity set)

• Entity Set Name: Student
• Attributes: Student_ID, Name, Age, Department
• Entity set contains all individual records:
  • Student 1 → (101, Ali, 20, CS)
  • Student 2 → (102, Ahmad, 21, Media)
  • Student 3 → (103, Ali, 19, Ecnomics)

Note:

• A Similar concept will be used for the weak entity set 
• No specific notations are used for the entity set

4. Associative Entity (Bridge Entity / Intersection Entity)

An Associative Entity is an entity that is created to represent a relationship between two other entities. It is mainly used to resolve Many-to-Many (M:N) relationships in ER modeling. Instead of directly linking two entities with an M:N relationship, we create a new entity (bridge) that connects them.

• Contains the primary keys of the related entities as foreign keys.
• Its composite key (combination of foreign keys) uniquely identifies each record.
• Add additional attributes describing the relationship (e.g., Grade, Date).
• Represented as a separate table in relational databases (bridge table).

Example: Three tables are required to explain 

• Student Table
• Course Table
• Enrollment Table (Associative Entity)

The following diagram explains the concept in detail

Explanation

• Student_ID in Enrollment → points to the Student table.
• Course_ID in Enrollment → points to Course table.
• Together, they form a composite primary key (no duplicate enrollment).
• Extra attributes like Semester, Enrollment_Date, and Grade describe the relationship.

Associative Entity Notation in ER Model:

Represented by a rectangle inside a diamond (or sometimes just a rectangle with rounded corners).

Explaination: Student{1}–{1..n}Enrollment{1..n}–{1}Course

A Student can enroll in many Courses. A Course can have many Students. Enrollment is the link between them, often storing extra details like grade or semester.

• Student {1} — {1..n} Enrollment

  • One Student can have many Enrollment records (because a student can enroll in multiple courses).

  • But each Enrollment record belongs to exactly one Student.

• Enrollment {1..n} — {1} Course

  • One Course can appear in many Enrollment records (because many students can enroll in the same course).

  • But each Enrollment record belongs to exactly one Course.

5. Recursive Entity

A Recursive Entity is an entity that has a relationship with itself. It means that records of the same entity set are related to each other.

• This usually happens when one role of an entity relates to another role of the same entity. Represented in ER diagrams as a looped relationship from the entity back to itself.

Example:  Employee Entity

• Attributes: Emp_ID, Name, Designation
• Recursive Relationship: An Employee can be a Manager of another Employee in the same entity set

Here, Manager_ID is a foreign key referencing Emp_ID in the same table.

Best practice: Keep Manager_ID nullable so that top-level employees (like CEO or department heads) can have NULL.

Recursive Entity Notation in ER Model:

Represented by a single rectangle (normal entity) with a relationship diamond connecting back to the same entity at both ends.

6. Super and Sub Entities

A Super Entity is a generalized entity in an ER model that represents the common features or attributes shared by two or more specialized entities (called Sub-Entities).

Instead of repeating the same attributes in multiple entities, those shared attributes are placed in a Super Entity, while the unique attributes of each specialized entity remain in their own Sub-Entities.

Key Points

• Avoids Duplication → Shared attributes are stored only once in the Super Entity.
• Parent-Child Relationship → The Super Entity is like a parent, and Sub-Entities are its children.
• Forms “Is-A Relationship” → Each Sub-Entity is a type of Super Entity.
• Inheritance → Automatically takes common attributes from the Super Entity.

Note: Super–sub entities (Overlapping Sub-Entities) is a case of Specialization in DBMS.

Example

Suppose we have two entities: Student and Teacher.

• Both have common attributes: Name, Email, Phone, and DOB.
• But they also have unique attributes:
  • Student → Roll_No, Course
  • Teacher → Employee_ID, Salary

Instead of repeating Name, Email, Phone, and DOB in both entities, we create a Super Entity:In this way, both Student and Teacher inherit the common attributes from the Super Entity Person. The following figure shows the detail 

Explanation

A case when a person (e.g., doing a PhD while also teaching as a Lecturer) can belong to both sub-entities at once and contains some common attributes as well.

• Ali (Person_ID = S_101) is both in the Student and Teacher tables → meaning he is a Student (PhD-Math) and also a Teacher (earning 100,000).

• Sabir (Person_ID = S_103) is only in the Student table → meaning she is only a student.

Super and Sub Entity Notation in ER Model:

A triangle (ISA symbol) is used to connect the super entity with its sub-entities.

7. Disjoint Sub Entities