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Week 9
October 24
• Modeling with ERD
R. Ching, Ph.D. • MIS • California State University, Sacramento
Entity Relationship (ER) Model
(applies to relational data model)
• High-level conceptual model
– Describes the structure of the database, and the
associated retrieval and update transactions on the
database
– Composed of
• Entity types
• Relationship types
• Attributes
R. Ching, Ph.D. • MIS • California State University, Sacramento
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ERD Notation
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Primary key
Relationship type
Relationship name
Entity type
Music_categories
Attributes
CDs
Classify
music_category_code {PK}
stock_number {PK}
music_category_title
1..1
0..* CD_title
artist
music_category_code
record_label_code
Multiplicity (constraint)
Degree of the Relationship: Binary
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD Notation
Alternatively
Entity type
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Primary key (underscored)
Relationship type
Relationship name
Music_categories
Attributes
Music_category_code
Music_category_title
Cardinality
CDs
Classify
Stock_number
CD_title
Artist
Music_category_code
Record_label_code
Zero
(circle)
Minimum
Maximum (inside) Many
(outside)
(crows feet)
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD
• Entity type – A group of objects with the same properties,
which are identified by the enterprise as having an
independent existence
• Relationship type – A set of meaningful associations
among entity types
• Attributes – A property of an entity or a relationship type
R. Ching, Ph.D. • MIS • California State University, Sacramento
5
Degree of Relationship Type
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• The number of participating entity types in a relationship
• Binary (two entity types)
Employee
Employee Awards
Receives
1..1
0..1
• Ternary (three entity types)
Net pay
Employee
Attribute of a relationship type
Emp_Benefits
Receives
Emp_Tax
R. Ching, Ph.D. • MIS • California State University, Sacramento
Recursive Relationship
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• A relationship type where the same entity participates more
tan once in different roles
Managers and staff are employees.
Manages
1..*
Manager
0..*
Staff are managed
by managers
A manager is an employee
Employee
Staff
Role name indicates the purpose an
entity type plays in a relationship type
R. Ching, Ph.D. • MIS • California State University, Sacramento
Attributes
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Property of an entity or relationship type
Customers
Cust_account Cust_name Cust_address Cust_phone Soc_Sec_Num
Customer_Accounts
Cust_account Current_balance
Credit_limit
Active_date
Expire_date
• Attribute domain
– Set of values that may be assigned to a single-valued
attribute
R. Ching, Ph.D. • MIS • California State University, Sacramento
Attributes of Attributes
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• Simple (atomic attributes) - composed of a single
component
• Composite - composed of multiple components
• Single valued - one value for an entity
• Multi-valued - one or more values for an entity
• Derived - value derived from a related attribute or set of
attributes
Student_ID FName MName
LName
Single-valued
Multi-valued Student_ID Semester Course_ID
More than one semester, more than one course_id
R. Ching, Ph.D. • MIS • California State University, Sacramento
Attribute Domain
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Customers Composite
Cust_account Cust_name Cust_address Cust_phone Soc_Sec_Num
Cust_first_name
Cust_last_name
John
William
Anita
Homer
Brown
Tell
Breake
Simpson
R. Ching, Ph.D. • MIS • California State University, Sacramento
• On an ER model,
should customer
name be shown as a
composite or simple
attribute?
• What is the attribute
domain of
Cus_name?
Derived Attributes
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• Derived - value derived from a related attribute or set of
attributes
Derived attribute
Student_ID Semester Course_ID Units Grade Grade_point
Student_ID Semester Course_ID Units Grade Grade_point
Student_ID Semester Course_ID Units Grade Grade_point
Units x Grade = Grade point
R. Ching, Ph.D. • MIS • California State University, Sacramento
Attributes as Keys
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Uniquely identifies an entity
Candidate key
Primary key
• Keys cannot change their values
(good for the life of the entity)
• An efficient means for identifying
an entity
• Alternate key - candidate that can also be used to access
an entity
• Composite key - composed of multiple attributes
(components)
R. Ching, Ph.D. • MIS • California State University, Sacramento
Attribute Diagrammatic Representation
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Customers
Composite attribute
Composite attribute
Cust_account {PK}
Cust_name
First_name
Middle_name
Last_name
Cust_address
Street_number
Zip_code (fk)
Cust_phone
Soc_sec_num
R. Ching, Ph.D. • MIS • California State University, Sacramento
Key
Foreign key
Attribute Diagrammatic Representation
Customer_Purchases
Composite key
Cust_account {PK}
Transaction_number {PK}
Date
Store_number
Payment_type
Foreign key
Foreign key
A customer may have more than one purchase
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Strong vs. Weak Entity Types
• Strong Entity Type – An entity type that is not existencedependent on some other entity type
– Often referred to as parent, owner or dominant entities
• Weak Entity Type – An entity type that is existencedependent on some other entity type.
– Often referred to as child, dependent or subordinate
entities
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Definition of a Weak Entity Type
“An entity type that borrows all or part of its primary key.
Identifying relationships indicate the entity types that
supply components of the borrowed primary key.”
Mannino, 1999
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Strong vs. Weak Entity Types
Customer_accounts
AccountNumber {PK}
FirstName
MiddleName
LastName
Address
ZipCode
MembershipDate
Customer_Purchases
AccountNumber {PK}
StoreNumber {PK}
Purchase
TransactionDate {PK}
1..1
0..*
TransactionNumber {PK}
PaymentType
Which is the strong and weak entity type?
How can you tell?
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Strong vs. Weak Entity Types
Customer_accounts
Customer_Purchases
Existent-dependence
AccountNumber {PK}
FirstName
MiddleName
LastName
Address
ZipCode
MembershipDate
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Also a FK
AccountNumber {PK}
StoreNumber {PK}
Purchase
TransactionDate {PK}
1..1
0..*
TransactionNumber {PK}
PaymentType
Which is the strong and weak entity type?
How can you tell?
R. Ching, Ph.D. • MIS • California State University, Sacramento
Attributes of Relationship Types
• Attributes produced through relationship types (i.e., not
retained in the entity types)
CustomerAccounts
AccountNumber
…
CustomerFlights
Reserve
1..1
AccountNumber {PK}
0..* TransactionDate {PK}
IntineraryNumber {PK}
…
NumberOfItineraries
R. Ching, Ph.D. • MIS • California State University, Sacramento
Derived attribute
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Structural Constraints
• Multiplicity – Number of possible occurences of an entity
type that may relate to a single occurrence of an associated
entity type through a particular relationship type
– One to one (1:1)
– One to many (1:*)
Defined by business rules
– Many to many (*:*)
• Cardinality and participation constraints
– Cardinality – Describes the maximum number of
possible relationship occurrences for an entity
participating in a given relationship type
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Cardinality and Participation Constraints
• Cardinality – Describes the maximum number of possible
relationship occurrences for an entity participating in a
given relationship type
• Participation – Determines whether all or only some entity
occurrences participate in a relationship
– 1:1 (1..1) minimum is one – mandatory participation
– 0:1 (0..1) minimum is zero – optional participation
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Cardinality
• 1:1 (one to one)
– Each entity in X is associated with at most one entity in
Y and conversely each entity in Y is associated with at
most one entity in X
• 1:* (one to many)
– Each entity in X can be associated with many entities in
Y but each entity in Y is associated with at most one
entity in X.
• *:* (many to many)
– Each entity in X can be associated with many entities in
Y and each entity in Y can be associated with many
entities in X.
R. Ching, Ph.D. • MIS • California State University, Sacramento
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Cardinality
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1:1 Relationships
Strong entity type
Weak entity type
Customers
Customer_ID {PK}
Customer_name
Customer_address
Zip_code
Accounts
Own
1..1
Mandatory
participation
A customer owns a
minimum and maximum
of one account
Account_number {PK}
1..1 Customer_ID {FK}
Account_type
Current_balance
An account is owned
by a minimum and
maximum of one
customer
Note. This would be avoided in the logical design, but could be implemented in
the physical.
R. Ching, Ph.D. • MIS • California State University, Sacramento
Cardinality
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1:M Relationships
Strong entity type
Weak entity type
Customers
Customer_ID {PK}
Customer_name
Customer_address
Zip_code
Accounts
Own
1..1
Mandatory
participation
A customer owns a
minimum one and
maximum of many
accounts
R. Ching, Ph.D. • MIS • California State University, Sacramento
Account_number {PK}
1..* Customer_ID {FK}
Account_type
Current_balance
An account is own by a
minimum and maximum
of one customer
Cardinality
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• *:* (many-to-many) relationship if a customer can own
more than one account (e.g., revolving, long-term), and
one account can have more than one owner (e.g., joint
account).
Customers
Customer_ID {PK}
Customer_name
Customer_address
Zip_code
Accounts
Own
1..*
A customer owns a
minimum one and
maximum of many
accounts
R. Ching, Ph.D. • MIS • California State University, Sacramento
Account_number {PK}
1..* Customer_ID {FK}
Account_type
Current_balance
An account is own by a
minimum of one
customer and maximum
of many customers
ERD Notation
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Primary key
Relationship type
Entity type
Relationship name
Video_categories
Attributes
Category_code {PK}
Category_title
Videos
Classify
1..1
0..*
Stock_number {PK}
Video_title
…
Category_code {FK}
Distributor_code {FK}
Minimum
Zero
Maximum
Many
For an occurrence of videos,
For an occurrence of video
there may exist one and only
categories, there may exist
one video category.
zero to many videos.
Cardinality
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD Notation
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Video_categories
Category_code {PK}
Category_title
Videos
Classify
1..1
0..*
Video categories classify videos
For an occurrence of distributors,
there are zero to many videos
Stock_number {PK}
Video_title
…
Category_code {FK}
Distributor_code {FK}
0..*
Release
1..1
Distributors
Distributors release videos
R. Ching, Ph.D. • MIS • California State University, Sacramento
Distributor_code {PK}
Distributor_name
ERD Notation
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Video_categories
Category_code {PK}
Category_title
Videos
Classify
1..1
Video categories classify videos
0..*
Stock_number {PK}
Video_title
…
Category_code {FK}
Distributor_code {FK}
0..*
Release
For an occurrence of videos, there is
one and only one video category
1..1
Distributors
Distributors release videos
R. Ching, Ph.D. • MIS • California State University, Sacramento
Distributor_code {PK}
Distributor_name
ERD Notation
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Video_categories
Category_code {PK}
Category_title
Videos
Classify
1..1
0..*
Stock_number {PK}
Video_title
…
Category_code {FK}
Distributor_code {FK}
0..*
Attribute of the
relationship type
Release
Number_of_videos_
released
1..1
Distributors
Distributor_code {PK}
Distributor_name
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD Notation
Alternate Notation
Entity type
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Primary key (underscored)
Relationship type
Relationship name
Music_categories
Attributes
Music_category_code
Music_category_title
Cardinality
CDs
Classify
Stock_number
CD_title
Artist
Music_category_code
Record_label_code
Zero
(circle)
Minimum
Maximum (inside) Many
(outside)
(crows feet)
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD Notation
Alternate Notation
Entity type
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Minimum cardinality of one
(a music category has to have at least one CD)
Music_categories
CDs
Music_category_code
Music_category_title
Classify
Weak entity type
(all four corners)
R. Ching, Ph.D. • MIS • California State University, Sacramento
Stock_number
CD_title
Artist
Music_category_code
Record_label_code
ERD Notation
Alternative Notation
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Music_categories
Music_category_code
Music_category_title
CDs
Classify
A record label is related to
a minimum of zero and
maximum of many CDs
Stock_number
CD_title
Artist
Music_category_code
Record_label_code
Produce
Record_labels
Record_label_code
Record_label
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD Notation
Alternative Notation
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Music_categories
Music_category_code
Music_category_title
CDs
Classify
A CD is related to a
minimum and maximum of
one record label
Stock_number
CD_title
Artist
Music_category_code
Record_label_code
Produce
Record_labels
Record_label_code
Record_label
R. Ching, Ph.D. • MIS • California State University, Sacramento
ERD Notation
Alternative Notation
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Music_categories
Music_category_code
Music_category_title
CDs
Classify
Quantity_produced
Attribute of a relationship
Stock_number
CD_title
Artist
Music_category_code
Record_label_code
Produce
Record_labels
Record_label_code
Record_label
R. Ching, Ph.D. • MIS • California State University, Sacramento
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R. Ching, Ph.D. • MIS • California State University, Sacramento