Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship...

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Lesson III Entity- Relationship Model

Transcript of Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship...

Page 1: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Lesson III

Entity-RelationshipModel

Page 2: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

IN THIS LESSON YOU WILL LEARN…

� What a ER diagram is (ERD) and what it is used for.

� To identify the main constructs in a ERD.

� To understand the difference between entities and entity-types and how to represent them.

� Definition of attributes, types and their representation.

� The meaning of relationship between entities and its representation.

� Attributes in a relationship.

� Maximum and minimum cardinality, types and representation.

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Page 3: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

INTRODUCTIONEntity-Relationship Model

� The entity-relationship model (ERM) is an abstract and conceptual representation of data.

� Diagrams created using this process are called entity-relationship diagrams (ERD).

� The ER modelling method was proposed by Peter Chen in 1976.

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Page 4: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

INTRODUCTIONEntity-Relationship Model

Graphical technique to model databases.

Entity-Relationship ModelIt is based on a perception of a real world that consists of a collection of objects called entities and relationships among these objects.

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Page 5: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ER DIAGRAMS (ERD)Example I

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Page 6: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Why do we want to draw a model for a part of the real world?

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ER DIAGRAMS (ERD)

We’ll start with thesimplest version of ER

and then extend it later on

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Page 8: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ER DIAGRAMS (ERD)Main Constructs

� The basic ER notation uses three mainconstructs:

– data entities (entity-types)

– attributes

– relationships

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ER DIAGRAMS (ERD)Exemple II

entity

relationshipattributes

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Page 10: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

DATA ENTITYDefinition

� Entities are the main objects of informationalinterest.

� A person, animal, place, object or event aboutwhich data is to be maintained.

e.g. students, subjects, departments, meetings, ...

o Represents a set or collection of objects in the real world that share the same properties.

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Page 11: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

DATA ENTITYEntity Instance

entity instances

data entity

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Page 12: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

DATA ENTITYEntity Instance

� A particular occurrence of a data entity is called an entity instance or an entityoccurrence .e.g. TEACHER is an entity.

“Xavier Morera” is an entity instance.

BUT…

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Page 13: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

DATA ENTITYRepresentation

� An entity is represented by a rectangle withthe name of the entity-type inside.

TEACHER

STUDENT

DEPARTMENT

SUBJECT

The type of the entity must be written in singular!13

Page 14: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ENTITYData Entity vs Entity Instance

Business

Computing

Modern languages

DEPARTMENT

entityentity instances

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Page 15: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

DATA ENTITYGuideline for Naming Entities

� An entity type name is a singular noun.

� An entity type should be descriptive and specific.

� An entity name should be concise.

� Event entity types should be named for theresult of the event, not the activity or process.

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ATTRIBUTESDefinition

� Named characteristics or properties of anentity-type.

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e.g. CAR � number_plate, brand, price, speed...

EMPLOYEE � id, name, address, course...

PRODUCT � code, name, price...

SALE � number, amount, method_payment...

Attributes can only take up a valueat any particular point in time!!

Page 17: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ATTRIBUTESDefinition

� An entity-type is described by a set of attributes that provide details about it.

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All entity instances of the same type have the same attributes!!

Page 18: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ATTRIBUTESGuideline for Naming Attributes

� An attribute name is a noun.

� An attribute name should be descriptive.

� An attribute name should be unique.

� Similar attributes of different entity types should use similar but distinguishing names.

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Page 19: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ATTRIBUTESRepresentation

� Attributes can be denoted by an ellipse withthe name of the property inside.

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Page 20: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ATTRIBUTESRepresentation

� There’s a simplier way to represent attributes.

� We’ll use single lines to connect the entitity to its attributes.

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Page 21: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.1Indentifying Entities and Attributes

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Is this entity-type correct?

Page 23: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

What’s the difference between an entity and a table?

Page 24: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ATTRIBUTESTypes

� There are two types of attributes:

- descriptors

- identifiers (or keys)

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Page 25: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ATTRIBUTESDescriptors vs Keys

o A descriptor is used to specify a nonuniquecharacteristic of a particular entity instance.

o An identifier (or key) is used to uniquelydetermine an instance of an entity.

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Page 26: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

KEYDefinition

� A key is an attribute (or combination of attributes) that uniquely identifies each instance of an entity type.

� An entity-type has at least one key.

� Some entities may have more than one key.

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Page 27: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

KEYDefinition

Which attribute/s make/s teachers different?

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Page 28: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

KEY Example

teacher with id=’34’

key

descriptors28

Page 29: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

KEYDefinition

� Some entities may have more than one key.

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key

key

Page 30: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

IDENTIFIERDefinition

� A key that has been selected as the unique identifying characteristic for an entity type.

OR…

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identifier

identifier

Page 31: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

IDENTIFIERExamples

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e.g. An identifier for CAR may be number_plate

An identifier for EMPLOYEE may be id

An identifier for PRODUCT may be code

An identifier for SALE may be number

Page 32: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

How many identifiers can an entity have?

Page 33: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

IDENTIFIER ATTRIBUTESTo Take Into Account…

� A unique identifier can be an attribute or a combination of attributes.

� Only one identifier has to be chosen for each entity.

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Page 34: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

IDENTIFIER ATTRIBUTESRepresentation

� Each attribute in the key is underlined in theER diagram, as shown:

identifier

descriptors

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Page 35: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.2Choosing Identifiers

Page 36: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Is an identifier a primary key in a table?

Page 37: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSDefinition

� Relationships represent real-world associationsamong instances of one or more entity-types.

e.g. A TEACHER teaches SUBJECTS

A teacher teaches subjects

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Page 38: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSGuideline for Naming Relationships

� Relationships name should be an active or a passive verb.

teacher subjects

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A teacher teaches subjects.A subject is taught by teachers.

Page 39: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSVisualization

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teacher subjects

� A relationship includes entity instances fromeach participating entity-type.

Page 40: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSVisualization

Xavi

Encarna

Joan

Database

Networks

Programming

TEACHER SUBJECT

teaches

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Xavi

Encarna

Joan

Page 41: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

How many instances can be involved in a relationship?

Page 42: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSIntroduction to Cardinality

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A teacher teaches subjects.A subject can be taught by teachers.

teacher subjects

How many??

Page 43: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSRepresentation

� A relationship is depicted by a diamond on theline that joins the entitity-types together.

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Page 44: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

How many relationships may there be between two entities?

Page 45: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSExample

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Page 46: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

ER DIAGRAMS (ERD)Indentifying the Objects

� An entity normally is the subject or the objectin the sentence.

e.g. A person has a car

A person has a car

� Relationships are usually denoted by verbs.

e.g. A person has a car

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Page 47: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.3Identifying Relationships

Page 48: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

How many entity-types can be involved in a relationship?

Page 49: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSDegree

� Degree of a relationship is the number of entity types that participate in it.

–Binary: between two entity types.–Ternary: among three entity types.–Unary: between an entity type.–Quaternary …

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Page 50: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSDegree

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Page 51: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.4Attributes and Relationships

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RELATIONSHIPSAttributes

In what subject ?

123, Steven, Barcelona, …, 8

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RELATIONSHIPSAttributes

We want to know the grade that each studenthas got for each subject.

Where do we have to put the attribute?

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Page 54: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSAttributes

Of what student ?

C3, Database analysis,… ,8

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Page 55: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSAttributes

123, Steven, … gets an 8… in C3, DB…

123, Steven, … gets a 5… in C5, Programming..

444, Karen, … gets a 3… in C3, Programming…

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Page 56: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSAttributes

… …

12, Steven, …

44, Karen,…

12, Steve… Database

Networks

Programming

student subjectenrols

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3

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Page 57: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.5The Vendors Database

Page 58: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

THE VENDORS DATABASE

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Page 59: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSCardinality

� It specifies maximum and minimum number of instances of an entity related to one instance of another entity.

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A teacher teaches subjects.A subject can be taught by teachers.

How many?

Page 60: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSCardinality

Given a teacher... how many subjects?

Given a subject... how many teachers?

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Page 61: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSCardinality

Xavi

Encarna

Joan

Database

Networks

Programming

TEACHER SUBJECT

teaches

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Page 62: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

RELATIONSHIPSCardinality

� There are two types of cardinality:

– Maximum cardinality (connectivity)

– Minimum cardinality

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Page 63: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

CARDINALITYExample

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given a teacher... ...how many subjects at most and how many at least?

given a department... ...how many teachers at most and how many at least?

Page 64: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MAXIMUM CARDINALITYDefinition

� It specifies the maximum number of instancesof an entity related to one instance of anotherentity.

“ one” or “ many”

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Page 65: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MAXIMUM CARDINALITYExample

Given a teacher... how many departments at most?

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one or many?

Given a department... how many teachers at most?

one or many?

Page 66: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MAXIMUM CARDINALITYExample

Given a teacher... how many departments?

Given a department... how many teachers?

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onemany

Page 67: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MAXIMUM CARDINALITYTo Remember

The maximum cardinality is represented on the oppositeside of the entity we areworking with.

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one

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MAXIMUM CARDINALITYER Representation

many � None � 1

many oneN 1

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Page 69: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MAXIMUM CARDINALITYTypes

� According to its maximum cardinality, relationships can be classified as either.

– “one to many” � 1:N

– “many to many” � N:M

– “one to one” � 1:1

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Relational modeling ideal

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MAXIMUM CARDINALITYExamples

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Page 71: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.6Maximum Cardinality

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Activity 3.7Entities vs Attributes

Page 73: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MINIMUM CARDINALITYDefinition

� It specifies the minimum number of instances of an entity related to one instance of anotherentity.

“ one” or “ none”

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Page 74: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MINIMUM CARDINALITYExample

Given a teacher... how many departments at least?

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one or none?

Given a department... how many teachers at least?

one or none?

Page 75: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MINIMUM CARDINALITYTo Remember

The minimum cardinality is represented by the entity weare working with.

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one

Page 76: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MINIMUM CARDINALITYExample

Given a teacher... how many departments?

Given a department... how many teachers?

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none one

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MINIMUM CARDINALITYER Representation

none � simple lineone � double line

onenone

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MINIMUM CARDINALITYExamples

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Page 79: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MINIMUM CARDINALITYExistence Contraint / Mandatory

existence constraint =

A department can’t exist

without an employee who manages it.

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TOTAL PARTICIPATION

Page 80: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

MINIMUM CARDINALITYOptional

existence constraint

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Total Participation(mandatory)

Partial Participation(optional)

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CARDINALITYRepresentation II

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e.g. given a teacher......how many subjects at most?...how many subjects at least?

(max, min)

Page 82: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.8MinimumCardinality

Page 83: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

CONSTRUCTING ERDGuidelines

� Before drawing the ER diagram, understand the requeriments.� Identify all potential entity-types.� Remove duplicate entities.

-Do not include the system as an entity-type!� List the attributtes of each entity.

-Ensure that the entity types are really needed.-Do not have attributes of one entity as attributes of anotherentity!

� Mark the primary keys.� Define the relationships.� Describe the cardinality and optionality of the relationships.� Remove redundant relationships.83

Page 84: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.9Bank

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Activity 3.10Nursery

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Activity 3.11Garage

Page 87: Lesson III Entity- Relationship Model - XTEC · Entity-Relationship Model The entity-relationship model (ERM) is an abstract and conceptual representation of data. Diagrams created

Activity 3.12Model Agencies

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HISTORICAL RECORDS

What’s the difference between both diagrams?88

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Activity 3.13General Elections