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Chapter 11. Enhanced ER modeling techniques Transparencies. Chapter 11 - Objectives. The limitations of the basic ER modeling concepts and the requirements to model more complex applications using enhanced data modeling concepts.

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Chapter 11 l.jpg

Chapter 11

Enhanced ER modeling techniques

Transparencies

© Pearson Education Limited, 2004


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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


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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


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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


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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


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The EER model

  • One of the most useful concepts associated with the EER model is called specialization/generalization.

© Pearson Education Limited, 2004


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Specialization/generalization

  • Associated with special types of entities known as superclasses and subclasses, and the process of attribute inheritance.

© Pearson Education Limited, 2004


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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


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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


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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


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AllStaff table holding details of all staff

© Pearson Education Limited, 2004


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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


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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


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Staff entity with subclasses representing job roles

© Pearson Education Limited, 2004


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Shared subclass and a subclass with its own subclass

© Pearson Education Limited, 2004


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Constraints on specialization/ generalization

  • Two constraints may apply to a specialization/generalization called participation constraints and disjoint constraints.

  • Participationconstraint

    • Determines whether every occurrence in the superclass must participate as a member of a subclass.

    • May be mandatory or optional.

© Pearson Education Limited, 2004


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Vehicle entity into vehicle types

© Pearson Education Limited, 2004


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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


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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



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Tables representing Staff and the Branch entities

© Pearson Education Limited, 2004


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Tables representing the Vehicle entity

© Pearson Education Limited, 2004


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