lt6-EERmodelling

7/29/2019 lt6-EERmodelling

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Database Systems&Applications

Lecture 6EER concepts(cntd.)


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EER Modeling

Sub classes, Super classes and Inheritance

Specialization and Generalization

Constraints and Characteristics of Specialization

and Generalization

Modeling of UNION Types Using Categories


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Specialization

The process of defining a set of subclass of anentity type (the super class of thespecialization).

The set of subclasses that form a specializationis defined on the basis of some distinguishingcharacteristics of the entities in the super

class.{SECRETARY, ENGINEER, TECHNICIAN} is a specialization of

EMPLOYEE based on thejob type of each entity.


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Specialization (cont.)

The same entity type may have severalspecializations based on different distinguishingcharacteristics.

The EMPLOYEE entity type may have two

specializations: Based on the methods of pay:

{SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}

Based on the type of job:

{SECRETARY, ENGINEER, TECHNICIAN}

A subclass can participate in specificrelationship type


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Diagrammatically representation ofspecialization in an EER diagram

Esuper

E1

E2

Specificattributes

Specificattributes


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Example

SECRETARY

d

TECHNICIAN ENGINEER

d

SALARIED_EMP

HOURLY_EMP

MANAGER

EMPLOYEE

MANAGES

PROJECT

BELONGS_TO

TRADE_UNION


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Specialization

The specialization process allows us to do thefollowing:

Define a set of subclass of an entity type

Establish additional specific attributes witheach subclass

Establish additional specific relationshiptypes between each subclass and otherentity types or other subclasses


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Generalization

Generalization is the reverse ofspecialization process. It defines a

generalized entity type from the givenentity types.


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Generalization (cont.)

CAR

LicensePlateNo

PriceMaxSpeed

VehicleID

NoOfPassengers

TRUCK

LicensePlateNo

PriceTonnage

VehicleID

NoOfAxies

VEHICLE

LicensePlateNoPriceVehicleID

d

CARMaxSpeed

NoOfPassengers

TRUCK Tonnage

NoOfAxies


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We can view {CAR, TRUCK} as aspecialization of VEHICLE

Alternatively, we can view VEHICLE as ageneralization of CAR and TRUCK


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Generalization suppresses the difference amongseveral entity types, identifying their commonfeatures, and generalize them into a single superclass of which the original types are special sub

classes.

The decision as to which process, generalization

or specialization, is more appropriate in aparticular situation is often subjective.


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Constraints onSpecialization/Generalization

Disjoint vs. Overlap Constraints

A total specialization vs. a partial

specialization

Specialization/Generalization hierarchiesand lattices

Utilizing specialization and generalizationin Conceptual Data Modeling


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Disjointness Constraint

Disjoint(d) constraint specifies that the subclasses of the specialization must be disjointed(an entity can be a member ofat most one of

the subclasses of the specialization)

In EER diagram, din the circle stands for

disjoint.


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Example

EMPLOYEE

Name SSN BirthDate Address

SECRETARY

d

TypeSpeed

TECHNICIAN

TGrade

ENGINEER

EngType

d

SALARIED_EMP

Salary

HOURLY_EMP

PayScale

Disjoint subclasses Disjoint subclasses


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Overlap Constraint

Overlap(o) specifies that the subclasses arenot constrained to be disjoint, i.e., the same(real-world) entity may be a member ofmorethan one subclass of the specialization.

Overlap is the default constraint and displayedby placing an o in the circle.


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Completeness constraint

Completeness constraint may be total or partial.

A total specialization constraint specifies that everyentity in the super class must be a member of some

subclass in the specialization.

Represented bya double line connecting the superclass to the circle.


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Completeness constraint(cont.)

A partial specialization allows an entity not tobelong to any of the subclasses, using a single line inEER.

e.g., if some EMPLOYEE entities, for example, sales

representatives, do not belong to any of the subclasses{SECRETARY, ENGINEER, TECHNICIAN}, then thespecialization is partial.

Represented bya single line connecting thesuperclass to the circle.


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Four Possible Constraints

The disjointness and completenessconstraints are independent.

There are four possible constraints on

specialization: Disjoint, total

Disjoint, partial

Overlapping, total Overlapping, partial


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Some insertion and deletion rulesapplied to specialization/generalization

Deleting an entity from a super class impliesthat it is automatically deleted from all thesubclasses to which it belongs

Inserting an entity in a super class implies thatthe entity is mandatorily inserted in allapplicable subclasses.

Inserting an entity in a super class of a total

specialization implies that the entity ismandatorily inserted in at least one of thesubclasses of the specialization.


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The differences between the specializationand generalization

The specialization process corresponds to a top-downconceptual refinement process during conceptualschema design.

we typically start with an entity type and then definesubclasses of the entity type by successivespecialization;

The generalization process corresponds to a bottom-up conceptual synthesis.

we typically start with an entity type of subclassesand then define super classes of the entity type bysuccessive generalization.

Specialization/Generalization hierarchies


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Specialization/Generalization hierarchiesand lattices


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Modeling of UNION Types UsingCategories

It is possible that single super class/subclassrelationship has more than one super classrepresenting different entity types. In this case, thesubclass will represent a collection of objects that is (a

subset of) the UNION of distinct entity types; we callsuch a subclass a union type or a category.

A category has two or more super classes that may

represent distinct entity types, whereas non-categorysuper class/subclass relationships always have a singlesuper class.


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Modeling of UNION Types Using Categories(cont.)

OWNER

U

PERSON

Address

Name

SSN

DriverLicenseNoBANK

BAddressBName

COMPANY

CAddressCName

OWNS

REGISTERED_VEHICLE

CYear

PurchaseDate

U

CAR TRUCK

LienOrRegular

CModel

CMake

CStyle

VehicelId

TMake TYear

TModel

Tonnage

VehicleId

M

N


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Modeling of UNION Types Using Categories(cont.)

Two categories in car registration database.

OWNER is a subclass of the union of PERSON, BANK, andCOMPANY

REGISTRERED_VEHICLE is a subclass of the union of CARand TRUCK

An entity that is a member of OWNER must exist inonly one of the super class.

Attribute inheritance works more selectively in the caseof categories.

e.g., each OWNER entity inherits the attributes of aCOMPANY, a PERSON, or a BANK, depending on the super

class to which the entity belongs.


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A Category can be total or partial?


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The superclass of a category may have different key attributes.

If a category is total (not partial), it may be representedalternatively as a specialization, and the choice of which

representation to use is subjective.

A category Or A Specialization

A t O A S i li ti


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A category Or A Specialization(cont.)

If the two classes represent the sametype of entities and share numerousattributes, including the same key

attributes, specialization/generalization ispreferred; otherwise, categorization ismore appropriate.

lname


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Aggregation

Used when we haveto model arelationshipinvolving (entitysets and) a

relationship set. Aggregation allows

us to treat arelationship set as anentity set for

purposes ofparticipation in(other) relationships.

Aggregation vs. ternary relationship:y Monitors is a distinct relationship,with a descriptive attribute. (i.e., until)y Also, can say that each sponsorshipis monitored by at most one employee.

budgetdidpid

started_on

pbudget

dname

until

DepartmentsProjects Sponsors

Employees

Monitors

lotssn

since


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