Super classes and Sub classes
As we discussed in Chapter 11, an entity type represents a set of entities of the same type
such as Staff, Branch, and Property For Rent. We can also form entity types into a hierarchy
containing super classes and sub classes.
Super class An entity type that includes one or more distinct sub groupings of its
occurrences, which require to be represented in a data model.
Subclass A distinct sub grouping of occurrences of an entity type, which require
to be represented in a data model.
Entity types that have distinct sub classes are called super classes. For example, the
entities that are members of the Staff entity type may be classified as Manager, Sales Personnel,
and Secretary. In other words, the Staff entity is referred to as the super class of the Manager,
Sales Personnel, and Secretary sub classes. The relationship between a super class and any one of its sub classes is called a super class/subclass relationship. For example, Staff/Manager
has a super class/subclass relationship.