Make a comparison between the strong entity and the weak entity.
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How are strong and weak entity sets represented in the E-R diagram? Explain with an example.
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When is the concept of entity used in data modelling?
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Define the terms owner entity, weak entity, identifying relationship, partial key.
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What is weak entity set and strong entity set?
Ans.
Sometimes an entity set may not have sufficient attributes to form a primary key. Such an entity set is termed a weak entity set. On the other hand, if any entity set has sufficient attributes to form a primary key, it is said to be strong entity set. For example, consider an entity set payment, which has three attributes payment_number, payment_date and payment_amount. Although, each payment entity is different, payments from different loans may share the same payment number. So, only on the basis of payment_number, we cannot distinguish between different entities. Thus, this entity set does not have a primary key, it is a weak entity set. Let us take an example of loan entity, with attributes loan_number and amount. Here loan_number behaves as a primary key for loan entity. So, loan is a strong entity set. This is given in fig. 1.8.
The concept of weak and strong entity sets are very closely related to the existence of dependencies. A member of a strong entity set is considered as a dominant entity, whereas a member of a weak entity set is considered as a subordinate entity.
The primary key of a weak entity set is formed by the primary key of the strong entity set, on which the weak entity set is existence dependent, plus the weak entity set discriminator. The discriminator of a weak entity set is a set of attributes that allow the distinction to be made among all those entities in the entity set that depends on one particular entity set. For example, the discriminator of the weak entity set payment is the attribute payment_number. Since a payment_number uniquely identifies one single payment for that loan, The discriminator of a weak entity set is also called the partial key of the entity set.
So, in the case of entity set payment, its primary key is (loan_number, payment_number), where the loan_number identifies the dominant entity of payment and payment_number distinguishes payment entities within the same loan. The identifying dominant entity set is said to own the weak entity set that identifies. The relationship that associates the weak entity set with an owner is the identifying relationship. In our example, fig. 1.8, loan_ payment is the identifying relationship for payment.
In an E-R diagram, a weak entity set is represented by a doubly outlined box and the corresponding identifying relationship by a doubly outlined diamond. In the fig. 1.8, a weak entity set payment is shown in the double outlined box and a link between the loan_payment relationship set and payment entity set is also double, and the loan_payment relationship set is also outlined doubly. The discriminator of weak entity set is dashed underlined as payment_number of entity set payment, while the primary key of a strong entity set is underlined with a solid line as loan_number in loan entity set.
Double lines in fig. 1.8 are used to identify the total participation. The participation of weak entity set payment in the relationship loan_ payment is total, meaning that every payment must be related via loan_payment to some account. An arrow from relationship set (loan payment) to strong entity set (loan) indicates that, each payment is for a single loan.
