Data Modeling with ER Diagrams
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What is an Entity-Relationship Model?
How is an E-R model represented as an E-R
diagram?
How can a video store be modeled as an E-R
diagram?
What is a role in a relationship type?
How can we record the history of video rentals?
How are cardinalities and related to roles?
How can we represent purchase orders?
How can we represent time cards and pay
statements?
Entity-Relationship Modeling
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An E-R model is a data model that includes
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Entity classes
Attributes of each class
Relationship types between classes
Constraints
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Types of attributes
Designation of key attributes
Cardinalities of relationship types
An E-R Model is typically represented
graphically
– E-R diagram, the technique we use
 UML diagram, an emerging standard for specifying
E-R models and software design
Entity Relationship
Diagrams
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Sample diagram for entity class Customer
Attribute
lastName firstName accountId
Key
Attribute
balance
otherUsers
Entity
Class
Customer
Multivalued
Attribute
numberRentals
Derived
Attribute
Composite
Attribute
address
street
city
zipcode
state
Component
Attribute
E-R Diagram for Class
Video
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Sample diagram for entity class Video
Key
Attribute
v ideotapeId
title
genre
Entity
Class
Video
Attribute
date
Acquired
length
rating
Relationship Types
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Example of representing relationship type
Owns between classes Store and Video
Enti ty
Cl ass
Al ternate
Rel ati onshi p
Nam e
Rel ati onshi p
T ype
IsOwnedBy
Store
Rel ati onshi p
Attri bute
Owns
purchase
Date
cost
Vi deo
Enti ty
Cl ass
Constraints on Relationship
Types
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Example of cardinality and participation
constraints
Mandatory
Optional
partic ipation
Cons traint
partic ipation
Cons traint
Is Ow nedBy
Store
Ow ns
1
V ideo
M
To-one purc has e c os t
Date
relations hip
role
to-many
relations hip
role
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Examples of current
Modeling Video
rentals and previous
Rentals
rentals
Differences are in
cardinalities and
M
1
Cus tomer
Rents
V ideo
attribute names
dateDue
c os t
dateRented
Cus tomer
M
Previously
Rented
date
Returned
M
c os t
dateRented
V ideo
Modeling Video Rentals as an
Entity Class
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Diagram shows entity class Rental and its
relationship types with Customer and Video
Note cardinalities and participation constraints
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A Rental entity cannot exist without being related
to both a customer and a video
Problem occurs because Rental has no key
attribute
1
Cus tomerHas
1
Rental
1
Has
M
dateDue
dateRented
c os t
V ideo
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A weak entity class is
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Weak Entity Classes
An entity class with no key of its own
An entity class whose entities cannot exist without
being related to other entities
An identifying relationship type is
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A relationship type that determines the keys of the
weak entities
Weak EntityOw ner Enti
Clas s
Clas s
1
Cus tomerHas
1
Rental
1
Has
V ideo
M
dateDue
dateRented
c os t
Identif y ing
Relations hip
Ty pe
Roles in Relationship Types
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Each entity in a relationship plays a specific
role in it
Roles are particularly important in relationship
types that relate an entity class to itself
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Each person in an IsChildOf relationship is either a
Role
parent or a child
Name
wi fe
ch i l d
1
Is
M a rri e d
To
M
Is
Ch i l d
Of
P e rso n
Is ParentOf
1
h u sb a n d
1 ..2
ssn
p a re n t
M In/max
Cardinality
Constraint
Employee Roles and
Cardinalities
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Is this an accurate model of the relationships between employees
and stores?
For example, does this diagram answer these questions correctly?
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Does a store have to have a manager?
Can an employee be the manager of more than one store?
What other questions can you ask about roles and cardinalities?
WorksIn
M
Paid
To
1
M
worker
Employee
1
Store
1
manager
Manage
s
1
1
M
PayStatement
M
Has
datePaid
TimeCard
amount
startTime
endTime
M
Has
Purchase Orders
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What does the diagram below mean?
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How do we buy many copies of a single movie?
Does each video have its own videoId before it is received?
What does it mean for an order of a video to have a quantity?
The problem with the diagram is with class Video
The diagram fails to distinguish between a movie and a copy of a
movie (a video)
We need to revise the diagram to show Movie as an entity class
id
1
Supplier
M
Has
PurchaseOrder
1
Orders
Video
M
quantity
Modeling Videos and Movies
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This diagram more accurately specifies the
true nature of Video and Movie
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It also has an accurate specification of
PurchaseOrder
purchas e
Date
Id
1
PurchaseOrder
Has
l i neNumber
M
PurchaseOrder
quantity
Detai l
M
uni tCost
M
Buys
From
Orders
vi deoId
1
1
movi eId
Suppl i er
title
Movi e
1
genre
name
address
l ength
rati ng
Is
Copy
M
Of
date
Acqui red
Vi deo
Time Cards and Pay Statements
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Classes TimeCard and PayStatement are weak
Does this diagram show the correct identifying
relationships and discriminators?
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For example, can an employee have 2 time cards
that begin at the same time but are at different
stores?
M
Employee
M
TimeCard
Has
1
Has
Store
1
1
startTime
endTime
Discriminator
Paid
To
M
PayStatement
Discriminator
datePaid amount