enhanced entity relationships model

7/29/2019 Enhanced Entity Relationships Model

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Enhanced Entity-Relationship Model


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Objectives2

In this class, student will learn: Limitations of basic concepts of the ER model and

requirements to represent more complex applications usingadditional data modeling concepts.

Most useful additional data modeling concepts of EnhancedER (EER) model called: specialization/generalization aggregation composition.

A diagrammatic technique for displayingspecialization/generalization, aggregation, and compositionin an EER diagram using UML.


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Enhanced Entity-Relationship Model

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Since 1980s there has been an increase inemergence of new database applications withmore demanding requirements.

Basic concepts of ER modeling are notsufficient to represent requirements of newer,more complex applications.

Response is development of additionalsemantic modeling concepts.


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The Enhanced Entity-Relationship Model

Semantic concepts are incorporated into theoriginal ER model and called the EnhancedEntity-Relationship (EER) model.

Examples of additional concepts of EER modelare: specialization / generalization

aggregation

composition


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Specialization / Generalization

Superclass/supertype An entity type that includes one or more distinct

subgroupings of its occurrences.

Subclass/subtype A distinct subgrouping of occurrences of an entity type.


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Generalization Hierarchies6

Employee

EmpNo

EmpName

EmpHireDate

...

SalaryEmp

EmpSalary

HourlyEmp

EmpRate

generalization hierarchysymbol

supertype

subtypes


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Superclass/subclass relationship isone-to-one (1:1).

Superclass may contain overlappingor distinct subclasses.

Not all members of a superclass needbe a member of a subclass.


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Attribute Inheritance

An entity in a subclass represents same real world object

as in superclass, and may possess subclass-specific

attributes, as well as those associated with the superclass.


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Specialization

Process of maximizing differences between members of anentity by identifying their distinguishing characteristics.

Generalization

Process of minimizing differences between entities byidentifying their common characteristics.


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AllStaff relation holding details of all staff


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Specialization/generalization of Staff entity into

subclasses representing job roles


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Specialization/generalization of Staff entity into

job roles and contracts of employment


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EER diagram with shared subclass and

subclass with its own subclass


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Constraints on Specialization /

Generalization

Two constraints that may apply to aspecialization/generalization:

participation constraints

disjoint constraints.

Participationconstraint

Determines whether every member in superclassmust participate as a member of a subclass.

May be mandatory or optional.


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Generalization

Disjoint constraint

Describes relationship between members of the

subclasses and indicates whether member of a

superclass can be a member of one, or more than one,subclass.

May be disjoint or nondisjoint.


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Generalization

There are four categories of constraints of

specialization and generalization:

mandatory and disjoint

optional and disjoint mandatory and nondisjoint

optional and nondisjoint.


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DreamHome worked example - Staff

Superclass with Supervisor and Manager

subclasses


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DreamHome worked example - Owner

Superclass with PrivateOwner and

BusinessOwner subclasses

ream ome wor e examp e erson


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ream ome wor e examp e - ersonsuperclass with Staff, PrivateOwner, and Client

subclasses


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EER diagram of Branch view of DreamHome

with specialization/generalization


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Aggregation

Represents a has-a or is-part-of relationship

between entity types, where one represents the

whole and the other the part.


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Examples of Aggregation


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Aggregation23

Sale

SaleDate

Employee

Item

Description

Cost

* *contains

Aggregation: the Sale consists of a set of Items being sold.


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Composition

Specific form of aggregation that represents an

association between entities, where there is a

strong ownership and coincidental lifetime

between the whole and the part.


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Example of Composition


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Composition

26Bicycle

Size

Model Type

Wheels

Rims

Spokes

1 2built from

Composition: aggregation where the components become the new object.

Crank

ItemID

Weight

Stem

ItemIDWeight

Size

1

1

1

1

Bicycle

Size

Model Type

Wheels

Crank

StemTwo ways to

display

composition.


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Enhanced Entity-RelationshipModel

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End of Presentation

enhanced entity relationships model

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