Page 104 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE
P. 104
Unit 6: Relational Language and Database Design
6.10 Weak Entity Sets Notes
An entity set attributes that does not have a primary key within them, is termed as a weak entity
set. As an example, consider the entity set Payment, which has the three attributes: payment -
number, payment - date and payment - amount, illustrated with the following E-R diagram:
Figure 6.8
Payment-date
loan-no amount
Payment-number payment-amount
Loan Payments
Loan Payment
A weak entity set Payment is indicated in E-R diagram by a doubly outlined box, because each
payment entity is district, payments for different loans may share the same payment number.
Thus, this entity set does not have a primary key, it is a weak entity set.
In the above figure, the weak entity set Payment is dependent on the strong entity set Loan (An
entity set that has a primary key is termed as a strong entity set), via the relationship loan-
payment identified by a double outlined diamond.
The below figure illustrates the use of double lines to indicate total participation. The
participation of the weak entity set Payment in the relationship loan-payment is total; it means
that every payment must be related via loan-payment to some account. The arrow from loan-
payment to loan indicates that each payment is for a single loan.
The discriminator of a weak entity set is underlined with a dashed, rather than a solid line. The
discriminator of the weak entity set Payment is the attribute payment-number, which is formed
by the primary key loan-number of the strong entity set Loan on which the weak entity set is
dependent. That is, in this case of the weak entity set payment, its temporary primary key
(discriminator) is {loan-number, payment-number}, where loan -number identifies the dominant
entity of a Payment, and payment - number distinguishes payment entities within the same
loan.
An entity set that has a primary key is termed as a strong entity set.
Example: The entity set loan in the above figure is a strong entity set with its primary
key loan-number.
6.11 Extended ER Features
6.11.1 Class Hierarchies
To classify the entities in an entity set into subclass entity is known as class hierarchies. Example,
we might want to classify Employees entity set into subclass entities Hourly-Emps entity set
LOVELY PROFESSIONAL UNIVERSITY 97
