Weak Entity Set
A weak entity set is an entity set
which does not have sufficient attributes to form a primary key. A strong
entity set is an entity set with primary key.
For example, the entity set payment has
three attributes: payment_number, payment_date and payment_amount.
Each payment entity is distinct, but payments for different loans may share
same payment number. Hence, this entity set does not have a primary key, and it
is a weak entity set.
A weak entity set must be associated with another
entity set, called the owner or identifying entity set. The weak
entity set is said to be existence dependent on the identifying
entity set. The relationship associating weak entity set with identifying
entity set is called identifying relationship.
In the above example, the identifying entity set
for payment is loan, and the relationship loan_payment
that associates payments and loan entities is an identifying
relationship. The discriminator of a weak entity set, also called
partial key of the entity set, allows distinguishing the entity
set that depends on a strong entity.
In E – R Diagram, a doubly outlined box
indicates a weak entity set, and a doubly outlined diamond indicates
identifying relationship.
Figure – 1: An illustration of E – R Diagram with weak
entity set.
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