The Entity Relationship Model

Sample Answer

Relationship Strength Types in Entity-Relationship Model

In the Entity-Relationship (ER) model, relationship strength defines how entities are connected and whether one entity's existence depends on another. The two primary relationship strength types are:

Identifying (Strong) Relationship

• Definition: The child entity's primary key includes the parent entity's primary key. The child cannot exist without the parent.
• ER Notation: Solid line connecting entities.
• Real-World Scenario: University Course Enrollment
  • Entities:
    • Department (parent): Primary key = department_id.
    • Course (child): Primary key = (department_id, course_code).
  • Relationship:
    • A Course must belong to a Department (e.g., "CS101" exists only under the Computer Science department).
    • The department_id is part of the Course's primary key, making this an identifying relationship.
  • Why Identifying?
    • Courses are defined within departments; a course cannot exist independently of its department.

Non-Identifying (Weak) Relationship

• Definition: The child entity has its own primary key and does not include the parent's primary key. The child can exist without the parent.
• ER Notation: Dashed line connecting entities.
• Real-World Scenario: Online Bookstore Orders
  • Entities:
    • Customer (parent): Primary key = customer_id.
    • Order (child): Primary key = order_id (independent of customer_id).
  • Relationship:
    • An Order is placed by a Customer, but an order record can reference a customer_id (foreign key) without being part of its primary key.