5-1

Chapter Five

Data Modeling with the

Entity-Relationship Model

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The Data Model

• A data model is a plan, or blueprint, for a database design.

• It is more generalized & abstract than a database design.

• It is easier to change a conceptual data model than it is to change an implemented database design.

• Modeling is not easy– You must abstract the important details of an

organization.

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Entity-Relationship Model

• The ER model consists of a set of graphical symbols that are used to create conceptual models of databases.

• There are several versions– Original E-R model — Chen (1976).– Extended E-R model — Chen model extensions.– Information Engineering (IE) — Martin (1990); it uses “crow’s

foot” notation.– IDEF1X — A national standard developed by the National

Institute of Standards and Technology.– Unified Modeling Language (UML) — The Object

Management Group; it supports object-oriented methodology.

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Entities

• Things that can be identified and that users want to track– Entity class — a collection of entities of a

given type– Entity instance — the occurrence of a

particular entity

• There are usually many instances of an entity in an entity class.

• Entities translate into database tables.

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CUSTOMER:The Entity Class and Two Entity Instances

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Attributes

• Attributes are an entity’s characteristics.

• All entity instances of a given entity class have the same attributes, but vary in the values of those attributes.

• Shown in Chen data models as ellipses.

• In crow’s foot model, they’re listed within a rectangular form.

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EMPLOYEE:Attributes in Chen

5-8

EMPLOYEE:Attributes in Crow’s Foot

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Identifiers

• Identifiers are attributes that uniquely identify entity instances.

• May be simple or composite.• Identifiers in data models become keys in database

implementations:– Entities have identifiers.– Tables (or relations) have keys.– We often use the terms interchangeably.

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Relationships

• Entities are associated with one another in relationships:– Relationship classes: associations among entity

classes– Relationship instances: associations among entity

instances

• A relationship class can involve two or more entity classes.– If two, it’s binary – If three, it’s ternary

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Binary Relationship

5-12

Ternary Relationship

DOCTOR MEDICINE

PATIENT

PRESCRIPTION

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Relationship Cardinality

• The maximum cardinality is the maximum number of entity instances that can participate in a relationship.

• The minimum cardinality is the minimum number of entity instances that must participate in a relationship.

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Maximum Cardinality

• Maximum cardinality is the maximum number of entity instances that can participate in a relationship.

• Three categories:– One-to-One [1:1]– One-to-Many [1:N]– Many-to-Many [N:M]

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The Three Types ofMaximum Cardinality

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Minimum Cardinality

• Minimum cardinality is the minimum number of entity instances that must participate in a relationship.

• Minimums are generally stated as either 0 or 1:– If 0, participation in the relationship by the entity is

optional• Ie, no entity instance must participate in the relationship.

– If 1, participation in the relationship by the entity is mandatory

• Ie, at least one entity instance must participate in the relationship.

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The Three Types ofMinimum Cardinality

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Data Modeling Notation

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Data Modeling Notation:ERwin

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Data Modeling Notation:N:M and O-M

Note that:(1) ERwin cannot

indicate true minimum cardinalities on N:M relationships

(2) Visio introduces the intersection table instead of using a true N:M model

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ID-Dependent Entities

• An ID-dependent entity is an entity whose identifier includes the identifier of another entity.

• The ID-dependent entity is a logical extension of the independent entity:– BUILDING : APARTMENT– PAINTING : PRINT

• The minimum cardinality from the ID-dependent entity is always one.

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ID-Dependent Entities

A solid line indicates an identifying

relationship

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Weak Entities

• A weak entity is an entity whose existence depends upon another entity.

• All ID-Dependent entities are weak.

• But not all weak entities are ID-Dependent.– The identifier of the parent does not appear in

the identifier of the weak child entity.

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Weak Entities (Continued)

A dashed line indicates a

nonidentifying relationship

Weak entities must be indicated by an

accompanying text box in Erwin – There is no

specific notation for a nonidentifying but weak

entity relationship

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ID-Dependent and Weak Entities

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Subtype Entities

• A subtype entity is a special case of a supertype entity:– STUDENT :

UNDERGRADUATE or GRADUATE

• The supertype contains all common attributes, while the subtypes contain specific attributes.

• The supertype may have a discriminator attribute that indicates the subtype.

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Subtypes with a Discriminator

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Subtypes: Exclusive or Inclusive

• If subtypes are exclusive, one supertype relates to at most one subtype.

• If subtypes are inclusive, one supertype can relate to one or more subtypes.

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Subtypes: Exclusive or Inclusive (Continued)

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Subtypes: IS-A relationships

• Relationships connecting supertypes and subtypes are called IS-A relationships, because a subtype IS A supertype.

• The identifer of the supertype and all of its subtypes must be identical, i.e., the identifier of the supertype becomes the identifier of the related subtype(s).

• Subtypes are used to avoid value-inappropriate nulls.

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ERwin Symbol Summary

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ERwin Symbol Summary (Continued)

5-33

David M. Kroenke’s Database Processing

Fundamentals, Design, and Implementation

(10th Edition)

End of Presentation:Chapter Five Part One