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The Enhanced ER Model

and Business Rules

MIS 122 Second Semester SY 2009 - 2010

Modern Database Management 8th Edition

Jeffrey A. Hoffer, Mary B. Prescott, Fred R. McFadden

1 2007 by Prentice Hall


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Objectives Definition of termsDefinition of terms

Use ofUse of supertypesupertype/subtype relationships/subtype relationships

Use of generalization and specialization techniquesUse of generalization and specialization techniques

Specification of completeness andSpecification of completeness and disjointnessdisjointness constraintsconstraints

DeveloDevelo su ert esu ert e/subt e hierarchies for realistic business/subt e hierarchies for realistic business

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situationssituations

Develop entityDevelop entity clustersclusters

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Supertypes and Subtypes

Subtype:Subtype: AA subgroupingsubgrouping of the entities in an entity typeof the entities in an entity typethat has attributes distinct from those in otherthat has attributes distinct from those in other

subgroupingssubgroupings

SupertypeSupertype:: A generic entity type that has a relationshipA generic entity type that has a relationshipwith one or more subtypeswith one or more subtypes

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Attribute Inheritance:Attribute Inheritance:

Subtype entities inherit values of all attributes of theSubtype entities inherit values of all attributes of thesupertypesupertype

An instance of a subtype is also an instance of theAn instance of a subtype is also an instance of thesupertypesupertype

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Figure 4-1 Basic notation for supertype/subtype notation

a) EERnotation

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b) MicrosoftVisio

Notation

Figure 4-1 Basic notation for supertype/subtype notation (cont.)

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Different modeling tools may have different notation for the samemodeling constructs


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Figure 4-2 Employee supertype with three subtypes

All employee subtypeswill have emp nbr,name, address, anddate-hired

Each employee subtype

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will also have its ownattributes


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Relationships and Subtypes

Relationships at theRelationships at the supertypesupertypelevel indicate thatlevel indicate thatall subtypes will participate in the relationshipall subtypes will participate in the relationship

The instances of aThe instances of a subtypesubtypemay participate in amay participate in arelationship unique to that subtype. In thisrelationship unique to that subtype. In this

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situation, the relationship is shown at the subtypesituation, the relationship is shown at the subtypelevellevel

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Figure 4-3 Supertype/subtype relationships in a hospital

Bothoutpatientsand residentpatients arecared for by aresponsible

physician

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Only resident patients areassigned to a bed


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Generalization and Specialization

Generalization:The process of defining a more generalentity type from a set of more specialized entity types. BOTTOM-UP

Specialization:The process of defining one or moresubtypes of the supertype and forming supertype/subtype

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re at ons ps. -

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Figure 4-4 Example of generalization

a) Three entity types: CAR, TRUCK, and MOTORCYCLE

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10All these types of vehicles have common attributes


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Figure 4-4 Example of generalization (cont.)

So we put

the shared

b) Generalization to VEHICLE supertype

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attri utes ina supertype

Note: no subtype for motorcycle, since it has no unique attributes


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Figure 4-5 Example of specialization

a) Entity type PART

Only applies tomanufactured arts

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Applies only to purchased parts


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b) Specialization to MANUFACTURED PART and PURCHASED PART

Created 2

Figure 4-5 Example of specialization (cont.)

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Note: multivalued attribute was replaced by anassociative entity relationship to another entity

subtypes


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Constraints in Supertype/ Completeness Constraint

Completeness ConstraintsCompleteness Constraints: Whether: Whether

an instance of aan instance of a supertypesupertypemustmustalso bealso bea member of at least one subtypea member of at least one subtype

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Partial Specialization Rule: No (single line)Partial Specialization Rule: No (single line)

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b) Partial specialization rule

A vehiclecould be acar, a

Figure 4-6 Examples of completeness constraints (cont.)

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ruc , orneither


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Constraints in Supertype/ Disjointness constraint

Disjointness ConstraintsDisjointness Constraints: Whether an instance: Whether an instanceof a supertype mayof a supertype may simultaneouslysimultaneouslybe a member of two (or more)be a member of two (or more)

subtypessubtypes

Disjoint Rule: An instance of the supertype can be only ONE of theDisjoint Rule: An instance of the supertype can be only ONE of thesubtypessubtypes

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subtypessubtypes

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a) Disjoint rule

Figure 4-7 Examples of disjointness constraints

A atient can either be

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outpatient or resident, but notboth


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b) Overlap rule

A part may be

Figure 4-7 Examples of disjointness constraints (cont.)

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both purchasedandmanufactured


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Constraints in Supertype/ Subtype Discriminators

Subtype DiscriminatorSubtype Discriminator: An attribute of the supertype: An attribute of the supertypewhose values determine the target subtype(s)whose values determine the target subtype(s)

DisjointDisjoint aa simplesimpleattribute with alternative values toattribute with alternative values toindicate the possible subtypesindicate the possible subtypes

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pertain to different subtypes. Each subpart contains apertain to different subtypes. Each subpart contains aboolean value to indicate whether or not the instanceboolean value to indicate whether or not the instance

belongs to the associated subtypebelongs to the associated subtype

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Figure 4-8 Introducing a subtype discriminator (disjointrule)

A simple attribute withdifferent possible

values indicating thesubtype

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Figure 4-9 Subtype discriminator (overlaprule)A composite

attribute withsub-attributesindicating yesor no todeterminewhether it is ofeach subtype

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Figure 4-10 Example of supertype/subtype hierarchy

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Entity ClustersEntity Clusters

EER diagrams are difficult to read when there are too many entitiesEER diagrams are difficult to read when there are too many entitiesand relationshipsand relationships

Solution: Group entities and relationships intoSolution: Group entities and relationships into entity clustersentity clusters

Entity clusterEntity cluster: Set of one or more entity types and associated: Set of one or more entity types and associatedrelationships grouped into a single abstract entity typerelationships grouped into a single abstract entity type

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Figure 4-13aPossible entityclusters for Pine

Valley Furniture inMicrosoft Visio

Related

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entitiescould

becomeclusters

03-enhanceermodel compatibility mode

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