2 the entity-relationship model

8/10/2019 2 the Entity-Relationship Model

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The Entity-Relationship ModelPart I.


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Database Design Stages

Application

Requirements

Conceptual

Design

Logical Design

Physical Design

Conceptual Schema

Logical Schema

Physical Schema


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Conceptual Design

What is Conceptual Design? Concise representation of the DB application requirements

Why Conceptual Design ?

It helps to understand application requirements better

It helps to communicate our understanding of application

It helps to come up with a gooddesign


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Conceptual Design (Contd)

Conceptual Models ER (Entity-Relationship)Model, Our focus

UML (Unified Modeling Language),

ORM (Object Role Modeling), etc.

ER Model Structures: Entitiesand Relationships

Constraints

An ER schema is represented using Entity-RelationshipDiagram (ERD)


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Modeling

A database can be modeled as: Collection of entities,

Relationship among entities.

An entityis an object that exists and is distinguishable fromother objects. Example: a person, company, event, bank account, store, etc.

Entities have attributes Example: a person have names and addresses

An entity set is a set of entities of the same type that sharethe same properties. Example: set of all persons, companies, trees, holidays

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ER: Entity Sets and Attributes

Entity set :Represent a set of objects Attribute:property of an entity, has a domain

In ER diagram

Entity setrectangle

AttributeOval.

Student

sNamesNumber

sAge

Entity Set Student

with attributes (sNumber, sName, sAge)


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Attributes Types

Primitive attributes: Attribute stores a single value (Number, String,Boolean, Date, )

Composite attributes: An attribute can be divided into sub-attributes

(each is primitive)

Multi-values attributes: Attribute with many values

Derived attributes: Attributes computed from others

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Student

sNamesNumber

sAge

These are primitive

attributes

DoB


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Complex Attributes

Composite Attribute: address Multivalued Attribute: major

major

statestreet

address

city

Derived Attribute: Age

Age

Studententity set

with all its attributes

Student

sNamesNumber

sAge

major

statestreet

address

city

Age

DoB


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Relationship Types

Relationship:Association (connection) between entities sets

Relationship Type:Class of relationships

Representation: Use a diamond shape

Student

sNumber

sName

Course

cNumber

title

Has

Taken

Relationship type HasTakento represent

Coursestaken by Students


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Relationships with Attributes

Student

sNumber

sName

Course

cNumber

title

Has

Taken

project

Relationship HasTakenhas an attribute projectwhich is the

project the Studentdid for the Course

Student

sNumber

sName

Course

cNumber

title

Has

Taken

A relationship may have attributes

These attributes do notbelong to any of the connected entities. But theybelong the relationship


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Example I (Simple Application)

Suppliershave a name, and location.Productshave a name, price, and number.Consumershave a name, and location.

Suppliers supply products on certain dates, while consumers buyproducts of certain quantity

How would you model this application?

a) What are the entities and attributes?b) What are the relationships?


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Modeling of Example I

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supplier

sName

sLoc

consumer

cName

cLoc

product

pNumber sNamesPrice

supplies buys

datequantity

This ER captures exactly what is written in text


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Example II (More Complicated)

Suppliershave a name, and location.

Productshave a name, and number.

Consumers have a name, and location.

SomeSuppliershave established contracts to supply a certain Productto a

particular Consumerfor specially negotiated price at a given quantity.

How would you model this application?


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Modeling of Example II

Supplier

sName

sLoc

Consumer

cName

cLoc

Supply

price

Product

pName pNumber

qty

Model the relationship Suppliersupplies Productsto Consumers

Ternary relationship

(three-way)


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Binary vs. Multi-way Relationships

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supplier

sName

sLoc

consumer

cName

cLoc

product

pNumber sName

supplies buys

pricequantity

Binary relationship


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Recursive Relationship Type:

Another Example An relationship type whose with the same participating entity

type in distinct roles

Example: the SUPERVISION relationship

EMPLOYEE participates twice in two distinct roles:

supervisor (or boss) role

supervisee (or subordinate) role

Each relationship instance relates two distinct EMPLOYEE

entities:

One employee in supervisorrole

One employee in superviseerole


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A RECURSIVE RELATIONSHIP

SUPERVISION

e1

e2

e3

e4

e5

e6

e7

EMPLOYEE

r1

r2

r3

r4

r5

r6

SUPERVISION

2

1

12

2

1

1

12

1

2

2


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ER Model so far

Entities and entity sets

Relationships

Binary, ternary, multi-way

Attributes

For entity sets and relationship types

Simple, composite, multi-valued, derived


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What about an Exercise

Assume you are building a database for a

Bank

What are the entity sets, attributes, and

relationships??

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E-R Diagram for a Banking Enterprise

2 the entity-relationship model

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