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Weak (identifying) relationships

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• A special property of the Cares-for relationship
  • Recall that the Dependent entity was a weak entity:

    There was no key attribute in the entity Dependent !!!

    Result: you can have multiple entities that have identical attribute values

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  • Look what happens in the Cares-for relationship:

    Using the care-for relationship information, we can now distinguish the "identically valued" dependents !!!

    The Cares-for relationship has provided some identifying information to each of the Dependent entity instances !!!

    In fact:

    Every dependent is now identifiable !!!         Mary, who is a daughter of John (SSN 123-45-6789) and Mary, who is a daughter of Kate (SSN 222-22-2222) can now be distinguished from one another !!!!

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• Weak entity and its (identifying) weak relationship

  • Weak relationship:

    Weak relationship = a relationship in which a weak entity obtains additional identifying information that make a weak entity identifiable

    (A weak relationship is sometimes called a identifying relationship for this reason)

    Example:

    The Care-for relationship is an example of a weak relationship.

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  • Important note:

    A Weak entity must participate totally in the weak relationship !!!! Because otherwise the weak entity can not be identified !!!

    So:

    the participation constraint on a weak entity in a weak relationship is always total

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  • Comment:

    A weak entity will no longer pose a problem to the database when we include the (identifying) information provided by the weak relationship