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 part