The Extended ER Diagram

by K. Yue

1. Introduction

• How do we select a modeling language? Three important criteria (there are many):
  1. Simplicity
  2. Expressiveness
  3. Basic model
• Using these metrics to rate ER and UML:
  1. Simplicity: ER simpler
  2. Expressiveness: UML more expressive
  3. Basic model
     • UML: object-oriented model
     • ER: somewhat object-oriented (more like object-based).
• Thus, you can imagine the basic ER model is too simplistic for many applications.
• 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:

See: https://en.wikipedia.org/wiki/Entity%E2%80%93relationship_model

OO Extensions:

• Generalization and specialization: Superclass and subclass. Two key concepts:
  1. Model 'a-kind-of' relationship.
  2. Do not use 'is-a' relationship to reason about it.
  3. Generalization can be Used to minimize redundant definitions of members and relationships in the subclasses.
• I assume that you are familiar with the generalization and specialization concept.
• When shall we use it?

Example: Figure 7.2 of Ricardo.

• 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.
• Generalization and specialization allows inheritance of members of a superclass by subclasses. It is not the only way in OOD. For example, GOLANG does not support inheritance.

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.
Bun is a person.