Example: Consider an entity set "LOAN” having attributes (Loan Number, Loan Amount, Customer ID) containing all the loans taken. The customer after taking a loan pay loan in installments. Consider the entity set "PAYMENT” representing all the payments made against all loans taken. The attributes of the "PAYMENT” are (Payment Number, Payment Date, Payment Amount). The Payment number refers to the number of payment made by the customer against a particular loan. The first payment made against all the loans will have payment number as 1. Second payment for any loan will have payment number as 2 and so on. We can see that there can be two entities in "PAYMENT” which have values for all attributes same and still they are two different entities. E.g. Suppose there are two loans having following values
Loan 1: (Loan Number= 1, Loan Amount= Rs. 2000, Customer ID = A)
Now Customer A makes his first payment on date 03/03/2010 of amount Rs100 and by chance B also makes his first payment on same date of same amount. So the two payment entities will have following values:
Payment by A: (Payment Number=1, date= 03/03/2010, Payment amount=Rs 100)
Payment by B: (Payment Number=1, date= 03/03/2010, Payment amount=Rs 100)
It means that no set attributes of "PAYMENT” will have unique values. So the entity set "Payment” will not have any key. Above mentioned two payments actually refers to different payments because they are made against different loan but in the "Payment” they doesn’t have any difference. Such Entity Sets which doesn’t have sufficient attributes to form a primary key are called Weak Entity Sets.
We should have relationships defined for weak entities which associate them with a strong entity in order to identify the different entities of these Entity Sets. Each entity of "Payment” is actually linked to some loan so the entity set "Payment” is dependent and "Loan” is Owner. The relationship "Loan-Payment” between "Loan” and "Payment” which associates loans with their payments is must to identify each payment in "Payment”. Such a relationship which links dependent entities of a weak entity set to their owners in strong entity set to identify the weak entities is called identifying relationship. Shown as doubly outlined diamond in ER Diagrams. Also Every Entity of a weak entity existence dependent on some entity of their owners. So every entity of a weak entity set must participate in the identifying relationship. Weak entity sets must have total participation in the identifying relationships. Shown by double lines. The following diagram represents the above described entity sets and relationships by an E-R Diagram.
The Weak entity sets may also be modeled as a multivalued, composite attribute of the owner entity set. Modeling as multivalued, composite attribute will be appropriate when weak entity set participate only in the identifying relationship and number of attributes are less. Otherwise modeling as Weak entity set will be more appropriate. A Weak Entity set may have several entities which have same values of all the attributes provided they are related to different strong owner entity. But all the weak entities related to a particular strong owner entity must be distinguishable. The set of attributes which allows making a distinction between the weak entities related to a particular strong entity is called the partial key or discriminator of the weak entity set.
