Question

List 5 characteristics of specialisation ​

Answer 1

Answer:

we may have several specializations defined on the same entity type (or superclass), as shown in Figure 8.1. In such a case, entities may belong to subclasses in each of the specializations. However, a specialization may also consist of a single subclass only, such as the {MANAGER} specialization in Figure 8.1; in such a case, we do not use the circle notation.

 

In some specializations we can determine exactly the entities that will become members of each subclass by placing a condition on the value of some attribute of the superclass. Such subclasses are called predicate-defined (or condition-defined) subclasses. For example, if the EMPLOYEE entity type has an attribute Job_type, as shown in Figure 8.4, we can specify the condition of membership in the SECRETARY subclass by the condition (Job_type = ‘Secretary’), which we call the defining predicate of the subclass. This condition is a constraint specifying that exactly those entities of the EMPLOYEE entity type whose attribute value for Job_type is ‘Secretary’ belong to the subclass. We display a predicate-defined subclass by writing the predicate condition next to the line that connects the subclass to the specialization circle.

Explanation: