Weak Entity Types

 

Weak Entity Types

Strong vs. Weak Entity Types

  • Strong entity types have their own key attribute that uniquely identifies each entity.

  • Weak entity types do not have a key of their own.

A weak entity is identified only through its relationship with another entity, combined with one of its attributes.

Owner (Identifying) Entity and Relationship

  • The entity type that helps identify a weak entity is called the:

    • Identifying (or owner) entity type

  • The relationship connecting them is called the:

    • Identifying relationship

📌 A weak entity cannot exist without its owner, so it always has:

  • Total participation (existence dependency) in the identifying relationship.

⚠️ However, existence dependency alone does not imply weakness.
Example:

  • DRIVER_LICENSE depends on PERSON

  • But DRIVER_LICENSE has its own key (License_number)

  • So it is not a weak entity

Example: DEPENDENT and EMPLOYEE

  • DEPENDENT is a weak entity type

  • Related to EMPLOYEE through a 1:N relationship

Attributes of DEPENDENT:

  • Name

  • Birth_date

  • Sex

  • Relationship (to employee)

Even if two dependents have identical attribute values, they are distinct entities if they belong to different employees.

➡️ A dependent is identified by:

(Employee + Dependent’s attribute value)

Partial Key (Discriminator)

  • A partial key uniquely identifies weak entities belonging to the same owner

  • It does not uniquely identify the entity by itself

Example:

  • DEPENDENT.Name can be a partial key

  • Assumption: No two dependents of the same employee share the same name

📌 If no single attribute suffices, a composite of all attributes may act as the partial key.

ER Diagram Representation

Weak entity types are visually distinguished in ER diagrams:

ComponentRepresentation
Weak entity        Double rectangle
Identifying relationship        Double diamond
Partial key        Dashed or dotted underline




Alternative Representation

Sometimes, weak entity types can be modeled as:

  • Composite multivalued attributes

Example:

  • DEPENDENT represented as a multivalued attribute Dependents of EMPLOYEE

Designer’s Choice Depends On:

✔ Whether the weak entity participates in other relationships
✔ Conceptual clarity and future extensibility

👉 If the weak entity has independent relationships, it is better modeled as a separate weak entity type.

Advanced Observations

  • A weak entity’s owner can itself be a weak entity

  • Multiple levels of weak entities are possible

  • A weak entity may:

    • Have multiple identifying entities

    • Participate in higher-degree identifying relationships

(These cases are discussed later in the chapter.)

Key Takeaways

✔ Weak entities lack a primary key
✔ Identified using an owner entity + partial key
✔ Always have total participation in identifying relationship
✔ Partial key is also called a discriminator
✔ Clear visual notation helps distinguish them in ER diagrams

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