Weak entities and total dependence Two entities participate in a relationship, and one entity is entirely dependent on the other for identification and existence. What must be present in the model?

Introduction / Context:
A weak entity cannot be uniquely identified by its own attributes alone and requires a relationship with an owning (strong) entity for identification. Modeling weak entities correctly is crucial when business rules dictate existence dependence, such as line items that only exist within orders or dependents that only exist within employee records.

Given Data / Assumptions:

• The dependent entity lacks a full identifier by itself.
• Its existence and identity are tied to a specific parent entity.
• A supporting identifying relationship is present.

Concept / Approach:

In ER modeling, a weak entity is paired with an identifying relationship to a strong entity. The primary key of the weak entity often includes the strong entity's key plus a partial discriminator. Total participation on the weak side is typical, reflecting existence dependence. This pattern maintains referential integrity and communicates that the child cannot stand alone.

Step-by-Step Solution:

Verification / Alternative check:

Standard ER texts depict weak entities with double rectangles and identifying relationships with double diamonds. The weak entity's key includes the owner's key, confirming dependence.

Why Other Options Are Wrong:

• Derived attribute: derivation does not resolve identification dependence.
• Optional participation: contradicts existence dependence (usually total participation is required).
• Multivalued attribute: orthogonal to the identification issue.
• Surrogate key: may hide, but does not change real-world dependence.

Common Pitfalls:

• Forgetting to enforce identifying relationships, leading to orphaned weak entities.
• Using arbitrary surrogate keys without modeling true business rules.

Final Answer:

A weak entity that depends on a strong (owner) entity