chapter 3: the enhanced e-r model © 2013 pearson education, inc. publishing as prentice hall 1...

CHAPTER 3:CHAPTER 3:THE ENHANCED E-R MODELTHE ENHANCED E-R MODEL

© 2013 Pearson Education, Inc.  Publishing as Prentice Hall1

Modern Database Management11th Edition

Jeffrey A. Hoffer, V. Ramesh, Heikki Topi

Chapter 3 © 2013 Pearson Education, Inc.  Publishing as Prentice Hall© 2013 Pearson Education, Inc.  Publishing as Prentice Hall

OBJECTIVESOBJECTIVES

Define termsDefine terms Understand use of supertype/subtype Understand use of supertype/subtype

relationshipsrelationships Understand use of specialization and Understand use of specialization and

generalization techniquesgeneralization techniques Specify completeness and disjointness constraintsSpecify completeness and disjointness constraints Develop supertype/subtype hierarchies for Develop supertype/subtype hierarchies for

realistic business situationsrealistic business situations Develop entity clustersDevelop entity clusters Explain universal (packaged) data modelExplain universal (packaged) data model Describe special features of data modeling Describe special features of data modeling

project using packaged data modelproject using packaged data model

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Basic E-R Model pushing 40 years old complex data relationships and new database

technology have outgrown it in some respects Enhanced E-R Model

a response to the shortcomings of the basic E-R model

not universally agreed upon in some respects introduced the supertype/subtype relationship

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SUPERTYPES AND SUBTYPESSUPERTYPES AND SUBTYPES Enhanced ER model: Enhanced ER model: extends original ER model extends original ER model

with new modeling constructswith new modeling constructs Subtype:Subtype: A subgrouping of the entities in an entity A subgrouping of the entities in an entity

type that has attributes distinct from those in other type that has attributes distinct from those in other subgroupingssubgroupings

Supertype:Supertype: A generic entity type that has a A generic entity type that has a relationship with one or more subtypesrelationship with one or more subtypes

Attribute Inheritance:Attribute Inheritance: Subtype entities inherit values of all attributes of the Subtype entities inherit values of all attributes of the

supertypesupertype An instance of a subtype is also an instance of the An instance of a subtype is also an instance of the

supertypesupertype

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Figure 3-1 Basic notation for supertype/subtype notation

a) EER notation

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Different modeling tools may have different notation for the same modeling constructs.

b) Microsoft

Visio Notation

Figure 3-1 Basic notation for supertype/subtype notation (cont.)

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Figure 3-2 Employee supertype with three subtypes

All employee subtypes will have employee number, name, address, and date hired

Each employee subtype will also have its own attributes

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RELATIONSHIPS AND SUBTYPESRELATIONSHIPS AND SUBTYPES

Relationships at the Relationships at the supertypesupertype level level indicate that all subtypes will indicate that all subtypes will participate in the relationshipparticipate in the relationship

The instances of a The instances of a subtypesubtype may may participate in a relationship unique to participate in a relationship unique to that subtype. In this situation, the that subtype. In this situation, the relationship is shown at the subtype relationship is shown at the subtype levellevel

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Use this type of relationship when either (or both) of the following are present: When there are attributes that apply to

some (but not all) of the instances of an entity type

When the instances of a subtype participate in a relationship unique to that subtype

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The property by which subtype entities inherit values of all attributes of the supertype.

This important property makes it unnecessary to include supertype attributes redundantly with the subtypes.

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Figure 3-3 Supertype/subtype relationships in a hospital

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

Specialization: The process of defining one or more subtypes of the supertype and forming supertype/subtype relationships. TOP-DOWN

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Figure 3-4 Example of generalization

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

All these types of vehicles have common attributes

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Figure 3-4 Example of generalization (cont.)

So we put the shared attributes in a supertype

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

b) Generalization to VEHICLE supertype

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Figure 3-5 Example of specialization

a) Entity type PART

Only applies to manufactured parts

Applies only to purchased parts

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b) Specialization to MANUFACTURED PART and PURCHASED PART

Note: multivalued composite attribute was replaced by an associative entity relationship to another entity

Created 2 subtypes

Figure 3-5 Example of specialization (cont.)

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Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype Total Specialization Rule: Yes (double line) Partial Specialization Rule: No (single line)

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Figure 3-6 Examples of completeness constraints

a) Total specialization rule

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b) Partial specialization rule

Figure 3-6 Examples of completeness constraints (cont.)

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Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes Disjoint Rule: An instance of the supertype

can be only ONE of the subtypes Overlap Rule: An instance of the supertype

could be more than one of the subtypes

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a) Disjoint rule

Figure 3-7 Examples of disjointness constraints

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b) Overlap ruleFigure 3-7 Examples of disjointness constraints (cont.)

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Subtype Discriminator: An attribute of the supertype whose values determine the target subtype(s) Disjoint – a simple attribute with alternative

values to indicate the possible subtypes Overlapping – a composite attribute whose

subparts pertain to different subtypes. Each subpart contains a Boolean value to indicate whether or not the instance belongs to the associated subtype

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Attribute of the supertype whose value determines to which subtype an instance belongs

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Figure 3-8 Introducing a subtype discriminator (disjoint rule)

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Figure 3-9 Subtype discriminator (overlap rule)

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Figure 3-10 Example of supertype/subtype hierarchy

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EER diagrams are difficult to read when there are too many entities and relationships.

Solution: Group entities and relationships into entity clusters.

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

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Figure 3-13a Possible entity clusters for Pine Valley Furniture in Microsoft Visio

Related groups of entities could become clusters

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Figure 3-13b EER diagram of PVF entity clusters

More readable, isn’t it?

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Figure 3-14 Manufacturing entity cluster

Detail for a single cluster

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Predefined data models Could be universal or industry-specific Universal data model = a generic or

template data model that can be reused as a starting point for a data modeling project (also called a “pattern”)

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ADVANTAGES OF PACKAGED ADVANTAGES OF PACKAGED DATA MODELSDATA MODELS

Use proven model componentsUse proven model components Save time and costSave time and cost Less likelihood of data model errorsLess likelihood of data model errors Easier to evolve and modify over timeEasier to evolve and modify over time Aid in requirements determinationAid in requirements determination Easier to readEasier to read Supertype/subtype hierarchies promote reuseSupertype/subtype hierarchies promote reuse Many-to-many relationships enhance model flexibilityMany-to-many relationships enhance model flexibility Vendor-supplied data model fosters integration with Vendor-supplied data model fosters integration with

vendor’s applicationsvendor’s applications Universal models support inter-organizational systemsUniversal models support inter-organizational systems

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Packaged data models are generic models that can be customized for a particular organization’s business rules.

Figure 3-15 PARTY, PARTY ROLE, and ROLE TYPE in a universal data model

(a) Basic PARTY universal data model

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Figure 3-15 PARTY, PARTY ROLE, and ROLE TYPE in a universal data model

(b) PARTY supertype/subtype hierarchy

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