What is Cardinality:
Cardinality in Entity-Relationship Diagrams (ERDs) defines the relationship between two entities in terms of how many instances of one entity can or must be associated with instances of another entity. It's a crucial concept in database design because it ensures the accurate modeling of real-world relationships between data tables.
Types of Cardinality:
One-to-One (1:1): One instance of an entity is related to only one instance of another entity.
- Example: A person can have only one passport, and a passport can belong to only one person.
One-to-Many (1:N): One instance of an entity is related to many instances of another entity.
- Example: A customer can place multiple orders, but each order is placed by only one customer.
Many-to-Many (M:N): Many instances of one entity can be related to many instances of another entity.
- Example: A student can take multiple courses, and a course can be taken by multiple students.
Explanation of the Diagram:
Entities: The diagram shows two entities: "Students" and "Courses."
Relationships: The relationship between "Students" and "Courses" is a "Many-to-Many" relationship.
Cardinality Notation: The cardinality is represented using the following notation:
Students (M): Many students can take a course.
Courses (N): A course can be taken by many students.
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