© Pearson Education Limited, 2004
1
Chapter 11
Enhanced ER modeling techniques
Transparencies
© Pearson Education Limited, 2004
2
Chapter 11 - Objectives The limitations of the basic ER
modeling concepts and the requirements to model more complex applications using enhanced data modeling concepts.
The main concepts associated with the Enhanced Entity–Relationship (EER) model called specialization/generalization.
© Pearson Education Limited, 2004
3
Chapter 11 - Objectives
A notation for displaying specialization/generalization in an EER diagram.
How to create tables that represent specialization/generalization in an EER model.
© Pearson Education Limited, 2004
4
The EER model Basic concepts are often perfectly
adequate for the representation of the data requirements for many different database applications.
However, basic concepts can be limiting when modeling more complex database applications with a large amount of data and/or data with complex interrelationships.
© Pearson Education Limited, 2004
5
The EER model
Stimulated need to develop additional ‘semantic’ modeling concepts.
Original ER model with additional semantic concepts is referred to as the Enhanced Entity–Relationship (EER) model.
© Pearson Education Limited, 2004
6
The EER model
One of the most useful concepts associated with the EER model is called specialization/generalization.
© Pearson Education Limited, 2004
7
Specialization/generalization Associated with special types
of entities known as superclasses and subclasses, and the process of attribute inheritance.
© Pearson Education Limited, 2004
8
Superclasses and subclasses Superclass
An entity that includes one or more distinct groupings of its occurrences, which require to be represented in a data model.
Subclass A distinct grouping of occurrences
of an entity type, which require to be represented in a data model.
© Pearson Education Limited, 2004
9
Superclass/subclass relationship
Superclass/subclass relationship is one-to-one (1:1).
Each member of a subclass is also a member of the superclass but has a distinct role.
© Pearson Education Limited, 2004
10
Superclasses and subclasses
We can use superclasses and subclasses to avoid describing different types of entities with possibly different attributes within a single entity.
Can also show relationships that are only associated with particular subclasses and not with superclass.
© Pearson Education Limited, 2004
11
AllStaff table holding details of all staff
© Pearson Education Limited, 2004
12
Attribute inheritance
An entity occurrence in a subclass represents the same ‘real world’ object as in the superclass.
Hence, a member of a subclass inherits those attributes associated with the superclass, but may also have subclass-specific attributes.
© Pearson Education Limited, 2004
13
Specialization/generalization Specialization
The process of maximizing the differences between members of an entity by identifying their distinguishing characteristics.
Generalization The process of minimizing the
differences between entities by identifying their common characteristics.
© Pearson Education Limited, 2004
14
Staff entity with subclasses representing job roles
© Pearson Education Limited, 2004
15
Shared subclass and a subclass with its own subclass
© Pearson Education Limited, 2004
16
Constraints on specialization/ generalization
Two constraints may apply to a specialization/generalization called participation constraints and disjoint constraints.
Participation constraint Determines whether every
occurrence in the superclass must participate as a member of a subclass.
May be mandatory or optional.
© Pearson Education Limited, 2004
17
Vehicle entity into vehicle types
© Pearson Education Limited, 2004
18
Constraints on specialization / generalization
Disjoint constraint Describes the relationship
between members of the subclasses and indicates whether it is possible for a member of a superclass to be a member of one, or more than one, subclass.
May be disjoint or nondisjoint
© Pearson Education Limited, 2004
19
Constraints on specialization / generalization
There are four categories of constraints of specialization and generalization: mandatory and disjoint optional and disjoint mandatory and nondisjoint optional and nondisjoint
© Pearson Education Limited, 2004
20
Creating tables to represent specialization/generalization
© Pearson Education Limited, 2004
21
Tables representing Staff and the Branch entities
© Pearson Education Limited, 2004
22
Tables representing the Vehicle entity
