DBMS Topics
Entity-Relationship ER Model
Last Updated : 21 May, 2026
The Entity-Relationship ER model is a high-level conceptual data model developed by Peter Chen in 1976. It provides a graphical representation of the database structure,
Introduction
The Entity-Relationship (ER) model is a high-level conceptual data model developed by Peter Chen in 1976. It provides a graphical representation of the database structure, making it easy to communicate the design to stakeholders before actual implementation.
The ER model is used during the database design phase and is later translated into a relational schema.
Core Concepts
1. Entity
An entity is a real-world object or concept that exists independently and about which we want to store information.
- Represented as a rectangle in ER diagrams
- Instances of an entity type are called entity instances or records
2. Attributes
An attribute is a property or characteristic that describes an entity.
- Represented as an ellipse (oval) connected to the entity by a line
Types of Attributes
| Type | Description | Notation |
|---|---|---|
| Simple | Cannot be subdivided (e.g., Age, Gender) | Regular oval |
| Composite | Can be subdivided (e.g., Name → First, Last) | Connected ovals |
| Single-valued | One value per entity (e.g., StudentID) | Regular oval |
| Multi-valued | Multiple values per entity (e.g., Phone numbers) | Double oval |
| Derived | Computed from other attributes (e.g., Age from DOB) | Dashed oval |
| Key | Uniquely identifies entity instances | Underlined in oval |
| Null | Can have no value (optional attribute) | Regular oval |
3. Relationships
A relationship is an association or connection between two or more entities.
- Represented as a diamond in ER diagrams
- The entities involved are connected to the diamond by lines
Degree of a Relationship
| Degree | Name | Description |
|---|---|---|
| 1 | Unary (Recursive) | Relationship between instances of the same entity |
| 2 | Binary | Relationship between two entities (most common) |
| 3 | Ternary | Relationship involving three entities |
Unary (Recursive)
Ternary
4. Cardinality and Participation
Cardinality specifies the number of instances of one entity that can be associated with instances of another entity through a relationship.
Participation Constraints:
- Total participation (mandatory): Every entity MUST participate — shown as double line
- Partial participation (optional): Some entities may not participate — shown as single line
Entity Sets vs. Relationship Sets
- Entity Set: Collection of entities of the same type (e.g., all students)
- Relationship Set: Collection of relationship instances of the same type (e.g., all enrollments)
Mapping ER to Relational Schema
| ER Concept | Relational Mapping |
|---|---|
| Entity set | Table (relation) |
| Attribute | Column |
| Key attribute | Primary key column |
| Multi-valued attribute | Separate table |
| Composite attribute | Multiple columns or sub-table |
| 1:1 relationship | Foreign key in either table |
| 1:N relationship | Foreign key in the N-side table |
| M:N relationship | New junction (bridge) table |
| Weak entity | Table + foreign key from identifying entity |
Extended ER Features
The Extended ER (EER) model adds:
- Specialization/Generalization — IS-A hierarchies
- Aggregation — Treating a relationship as an entity
- Categories/Union Types — Entity from multiple superclasses
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