ER Model 1

Chapter 3:The Entity Relationship Model

Agenda• Basic Concepts of the E-R model (Entities,

Attributes, Relationships)• Basic Notations of the E-R model

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Elements of the E-R Model

• E-R model was introduced by Peter Chen in1976

• Key elements:– Entities– Attributes– Identifiers– Relationships

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Entities

• An entity is something that can beidentified in the real world.

• Examples:Peter Johns, MIS, 2, 12345May Carson, ACCT, 54678Ray Charles, 1134, CS, Prof

• Entities of a given type are grouped intoentity classes.

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Entities (2)

STUDENTName Major Year IDAlan John MIS 2 12345May Carson ACCT 1 54678

FACULTYName StaffID Dept PostRay Charles 1134 CS Prof

...

...

...

Entit

y Cl

ass

Entit

y In

stanc

es

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Attributes

• Entities have attributes that describe theentity’s characteristics.

• Types of attributes: single-valued, multi-valued, composite. Examples are:

Single-valued: “Year 2”Multi-valued: {MIS, Marketing}Composite: (FirstName, LastName)

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Relationships

• A relationship indicates how one or moreentity classes interact with one another.

• Each entity plays a role in a relationship.• Degree of the relationship: number of

entities in a relationship.• Understanding the relationship is important

because it will affect how the database isconstructed and used.

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E-R Concepts

• entity type = entity class = entity• entity instance = entity occurrence = record• entity type: a set of objects in the real world

with the same properties (not the samevalues) with a physical or conceptualexistence

• entity instance: each entity has its ownvalues for each attribute

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Sample E-R Diagram

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Basic E-R Notation

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Basic E-R Notation

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Basic E-R Notation

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E-R Model Constructs

• Strong versus Weak Entity Type– Independent versus dependent entity.– Identifying owner.– Identifying relationship.– Weak entity identifier is its partial identifier

combined with that of its owner.

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E-R Model Constructs

• Attribute - property or characteristic of anentity type

• Simple versus Composite Attribute

• Single-Valued versus Multivalued Attribute

• Stored versus Derived Attributes

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E-R Model Constructs

• Identifier or Key - An attribute (orcombination of attributes) that uniquelyidentifies individual instances of an entitytype.

• Simple Key versus Composite Key• Candidate Key

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E-R Model Constructs

• Criteria for Selecting Identifiers– Will not change in value.– Will not be null.– No intelligent identifiers (containing e.g.

locations or people that might change)– Substitute new, simple keys for long, composite

keys.

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Composite Attribute

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Entity with a multivalued attribute (Skill) and derivedattribute (Years_Employed)

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Simple and composite key attributes

(a) Simple key attribute

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(b) Composite key attribute

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Stored vs Derived Attributes

• Derived attribute: the value of an attribute isderived from something else

STUDENT

SIDName

FName LName

Majors

DOB

Age

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Derived Attributes

• Example 1: Age = today date - Date_Birth

• Example 2: total number of staff can becalculated by counting the total number ofstaff entity instances

• Example 3: deposit = 2 * rent

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BRANCHBranch_No

Fax_NoAddress

Total_Branch

Street City ZipCode

Tel_No

How to Interpret this Model?

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Relationships

• Definition: a meaningful association amongentity classes

• “Meaningful” implies that the relationshipallows us to answer questions that could notbe answered given only the entity classes

• In general, any number of entity classesmay participate in a relationship

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Relationships

• Relationship Type versus Instance

• Degree of a Relationship - number of entity types

that participate in it.

– Unary (or Recursive) Relationship

• Bill-of-Materials

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Relationship types and instances(a) Relationship type (Completes)

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(b) Relationship instances

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Relationships• Degree of Relationship

– Unary Relationship– Binary Relationship– Ternary Relationship

• Attributes of Relationships– Many-to-Many– Associative Entities (Gerunds)

• All relationships involved are “many”• Result has independent meaning• Gerund has one or more non-key attributes

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Relationships of different degrees

(a) Unary relationships

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Role Names

• Unary relationship: the same entity classparticipates more than once in different rules

EMP

supervisor

supervisee

Supervises

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(b) Binary relationships

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Role Names

• Role names may also be used when twoentity classes are associated through morethan one relationships

EMP DEPT

manages

works_for

manager

member of staff

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(c ) Ternary relationships

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Ternary Relationship• Statement: venders can supply various parts

to warehouses

• Ternary relationship ≠ 3 binary relationships

PART

VENDORsupplies

WAREHOUSE

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4-nary Relationship

Solicitor

Financial_InstitutionBuyer

Bid

arranges

• A buyer, advised by a solicitor, and supported by a financial institution, places a bid for a property• A simultaneous relationship among four instances