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Weak Entity Sets

  • Weak Entity Set: An entity set whose members owe their existence to some entity in a strong entity set.
  • entities are not of independent existence.
  • each weak entity is associated with some entity of the owner entity set through a special relationship.
  • weak entity set may not have a key attribute.

Weak Entity set- Example

  • We depict a weak entity set by double rectangles.
  • We underline the discriminator of a weak entity set with a dashed line.
  • payment_number – discriminator of the payment entity set
  • Primary key for payment – (loan_number, payment_number)


Note: the primary key of the strong entity set is not explicitly stored with the weak entity set, since it is implicit in the identifying relationship.


If loan_number were explicitly stored, payment could be made a strong entity, but then the relationship between payment and loan would be duplicated by an implicit relationship defined by the attribute loan_number common to payment and loan.

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