In this article Entity Relationship Diagrams we give the information about An ER diagram is a visual representation of entities, attributes, and relationships in a database.
ER Diagrams (Entity-Relationship Diagrams):
An ER diagram is a visual representation of entities, attributes, and relationships in a database. It is used in the conceptual design phase of a database to model the real-world scenario in a structured and organized way.
Purpose of ER Diagrams
- To represent data requirements in a graphical format
- To assist in the design and planning of a database
- To define entities, relationships, and constraints
- To provide a clear communication tool between database designers and stakeholders
Basic Components of ER Diagrams
- Entity
Represents a real-world object.
- Symbol: Rectangle
- Example: Student, Book, Department
- Entity Set
A collection of similar entities.
- Attributes
Properties of an entity or relationship.
| Type of Attribute | Symbol | Example |
| Simple | Oval | Name, Age |
| Composite | Oval with sub-ovals | Name → First, Last |
| Multivalued | Double Oval | Phone Numbers |
| Derived | Dashed Oval | Age from DOB |
| Key Attribute | Underlined Oval | Roll No |
- Relationship
Association between entities.
- Symbol: Diamond
- Example: enrolls in, borrows, works for
- Relationship Types
| Type | Description | Example |
| One-to-One (1:1) | One entity maps to one | One student has one library card |
| One-to-Many (1:N) | One maps to many | One teacher teaches many students |
| Many-to-Many (M:N) | Many to many | Students enroll in many courses |
- Weak Entity
- Symbol: Double rectangle
- Cannot exist without a strong entity
- Requires a discriminator/partial key
- Participation Constraints
- Total Participation: Double line (entity must participate)
- Partial Participation: Single line (entity may participate)
- Generalization & Specialization
- Generalization: Combining entities into a higher-level entity
- Specialization: Splitting an entity into sub-entities
Example ER Diagram: Student-Course Enrollment
Scenario:
- A Student enrolls in a Course
- Each Course has a Course ID and Title
- The Enrollment has attributes like Date and Grade
ER Diagram:
+————+ +————+
| Student | | Course |
+————+ +————+
| Roll No | | Course ID |
| Name | | Title |
+————+ +————+
\ /
\ +———–+ /
\——–| enrolls |<—/
+———–+
| Date |
| Grade |
+———–+
Symbols Summary Table
| Component | Symbol |
| Entity | Rectangle |
| Weak Entity | Double Rectangle |
| Attribute | Oval |
| Key Attribute | Underlined Oval |
| Multivalued | Double Oval |
| Derived | Dashed Oval |
| Relationship | Diamond |
| Total Participation | Double Line |
| Partial Participation | Single Line |
Advantages of ER Diagrams
- Clear visual structure
- Easy to convert to relational schema
- Simplifies complex database design
- Useful for requirement gathering and communication
Limitations
- Not suitable for dynamic behavior (e.g., transactions, procedures)
- May become cluttered for very large systems
Common Use Cases
- University Management System
- Hospital or Patient Database
- Library Management System
- Inventory or Sales System
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