Weak Entity Types

• Definition:

  • Weak entity types are those without their own key attributes, unlike strong entity types that possess key attributes.

• Identification of Weak Entities:

  • Weak entities depend on an identifying (owner) entity type for their identification.
  • They are identified by their relationship to a specific entity from another type, combined with their attribute values.

• Identifying Relationship:

  • The relationship connecting a weak entity type to its owner entity type is termed the identifying relationship.

• Total Participation Constraint:

  • Weak entities have a total participation constraint with respect to their identifying relationship; they cannot exist without being linked to an owner entity.
  • Note: Not every existence dependency leads to the creation of a weak entity type.
  • Example: A DRIVER_LICENSE requires a PERSON but is not a weak entity since it has its own key (License_number).

• Example of Weak Entity:

  • DEPENDENT entity linked to EMPLOYEE.
  • Attributes of DEPENDENT:
    • Name (first name of the dependent)
    • Birth_date
    • Sex
    • Relationship (to the employee)
  • Despite the potential for multiple DEPENDENTs to have the same attribute values, they are distinct when associated with a specific EMPLOYEE.

• Ownership:

  • Each employee is considered to own the dependents linked to them.

• Partial Key:

  • A weak entity type usually has a partial key to uniquely identify its entities related to the same owner.
  • Example: Assume no two dependents of the same employee share the same first name; thus, the attribute Name for DEPENDENT serves as the partial key.

• ER Diagram Representation:

  • In ER diagrams, weak entity types and their identifying relationships are depicted with double-lined boxes and diamonds.
  • The partial key attribute is illustrated underlined with dashed lines.

• Complex Attributes:

  • Weak entity types can also be shown as composite or multivalued attributes.
  • Example: A multivalued attribute Dependents for EMPLOYEE can encompass attributes like Name, Birth_date, Sex, and Relationship.

• Design Considerations:

  • Database designers must decide on the representation based on the weak entity's participation in relationships beyond its identifying relationship type.

• Hierarchical Weak Entities:

  • Multiple levels of weak entity types can be defined where an owner entity type might also be a weak entity, and a weak entity can have multiple identifying entity types and relationships more complex than degree two.

• Terminology:

  • Identifying entity type: parent or dominant entity type.
  • Weak entity type: child or subordinate entity type.
  • Partial key: also known as the discriminator.