The Extended ER Diagram

by K. Yue

1. Introduction

• The basic ER model is too simplistic for many applications.
• Furthermore, the concept of entity is not as well defined as class. It may be regarded as somewhat object-based, but not object-oriented.
• Results: all kinds of extensions.
• Be careful when you use these extensions.
  • Many versions
  • Some conflicting

2. Extensions

• Most of these extensions are based on the object-oriented model.
• As you use more and more of them, consider using an OO modeling language.
• EA-RA model: relationship may have attributes.

Example:

Drawn using https://www.draw.io/:

• Most ER extension provides for attributes for relationships.

Example:

An ER diagram using Chen's notation.

OO Extensions:

• Generalization and specialization: Superclass and subclass. Two key concepts:
  1. Model 'a-kind-of' relationship.
  2. Generalization can be Used to minimize redundant definitions of members and relationships in the subclasses.
• Note that many use the term subclass, subtype or sub-entity in a similar manner. They are not entirely the same.
• I assume that you are somewhat familiar with the generalization and specialization concept.
• When shall we use it?

Example: Figure of Hoffer:

• Property of disjointness of subclasses:
  • disjoint: an object cannot belong to two subclasses at the same time.
  • overlapping: an object can belong to two subclasses at the same time.
• Beware of overlapping subclasses. Recommendation: avoid overlapping subclass in ER modeling.

Example:

Three entities, person, faculty and student (Figure 7.4 of Ricardo) are modeled as superclass and subclass. What is an alternative model?

• Completeness of specialization: a superclass object must be an object of one of the subclasses: total specialization.
• In general, complete and disjoint specialization is preferred.

Example:

Figure 7.2 of Ricardo. Give an example of a partial specialization.

• Multiple inheritance: in general, do not use it.

Example:

Figure 7.4 of Ricardo. What do you think about TA being a subclass of both faculty and student?

• In general, beware of ambiguity in English.

Examples:

John has a car.
A car has an engine.
A man is a person.
A man is a kind of a person.
A student is a person.