Web-DSS-Chapter-03

58 CHAPTER 3 ■ Entity-Relationship Modeling
Attributes
common to all
entities
SUPERCLASS
General
entity type
SUBCLASS 1 SUBCLASS 2 SUBCLASS N
Att. of
subclass1
Att. of
subclass2
Specialized
entity type
Figure 3.23 The notations for superclass and subclass relationships.
There are several reasons for introducing superclass and subclasses into an E-R model.
First, incorporating a superclass and subclasses maintains the cleanliness of the structure and
improves the conceptual flow. Second, it adds familiar semantic information to an E-R diagram.
We consider using subtypes if the following conditions apply:
1. Certain attributes apply to some, but not all, instances of an entity type.
2. An instance of a subtype participates in a relationship unique to that relationship (see
Figure 3.24).
Definition
The Enhanced Entity-Relationships (EE-R) model is a revised E-R model that extends the original
E-R model and supports additional semantic concepts by providing new modeling constructs.
A superclass is an entity type that has one or more distinct sub groups with unique attributes.
A subclass is an entity type that shares common attributes or relationships distinct from other
subclasses.
3.15.2 Attribute Inheritance and Subclass Relationships
Attribute Inheritance is the property by which subclass entities inherit attributes of the superclass.
The subclass entity type is an entity type in itself with a set of attributes and relationships.
In addition to its own attributes, a subclass entity inherits all the attributes of its superclass. For
example, the STUDENT subclass has attributes such as MajorDept, Class, and GPA. It also in-