10.1 What is a Weak Entity?

An entity type that does not have a primary key of its own is called a Weak Entity Type.

• Owner Entity: The entity type on which the weak entity depends.
• Identifying Relationship: The relationship that links the weak entity to its owner.

10.2 Characteristics of Weak Entities

1. Existence Dependency: If the owner entity is deleted, the weak entities are automatically deleted.
2. Total Participation: A weak entity always has total participation in its identifying relationship.
3. Partial Key (Discriminator): A set of attributes that can uniquely identify weak entities that are related to the same owner entity.

• Notation: Dashed underline.

10.3 Example: Dependents of an Employee

• Owner Entity: EMPLOYEE (Key: EmpID).
• Weak Entity: DEPENDENT (Attributes: Name, BirthDate).
• Partial Key: Name. (Two different employees might have a dependent named 'Aryan', but one employee likely won't have two dependents with the exact same name).
• Primary Key of Weak Entity: The combination of {Owner's Primary Key + Weak Entity's Partial Key}. In this case: {EmpID, Dependent_Name}.

10.4 Diagram Summary