The Entity–Relationship (ER) Model, proposed by Peter Chen, is a conceptual data model used to design and visualize a database. It represents real-world entities and the relationships between them.
An object that exists in the real world and has distinguishable characteristics.
Examples: Student, Employee, Product.
Strong Entity — has its own primary key
Weak Entity — depends on another entity
Properties of an entity.
Types:
Simple (Name)
Composite (Full Address)
Derived (Age from DOB)
Multivalued (Phone numbers)
Key Attribute (RollNo)
Associations between entities.
One-to-One (1:1)
One-to-Many (1:M)
Many-to-Many (M:N)
Example:
Student — Enrolls — Course
| Concept | Symbol |
|---|---|
| Entity | Rectangle |
| Weak Entity | Double rectangle |
| Attribute | Oval |
| Key Attribute | Underlined oval |
| Relationship | Diamond |
| Multivalued Attribute | Double oval |
Student — Enrolls — Course
Student (SID, Name, Age)
Course (CID, Title)
Relationship: Enrolls (Date)
The ER Model provides a clear and structured method for designing a database at the conceptual level. It helps convert real-world problems into a database blueprint, which is later transformed into relational tables.
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