Post on 17-Dec-2015
Data Modeling and Entity-Relationship Model II
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I want a database to maintain departments in my company. Store
information about my employees, their projects and assignments. I want ….
$$$
User requirement
A database
YOUR JOB!
Data Modeling (Ch.4)
Database Design (Ch.5)
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Overview of Chapter 4: Data Modeling
• Input: User requirement; Output: E-R Diagram• Use Entity-Relationship Diagram (E-R Diagram) to capture all user
requirements
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Review of Previous Class: Draw an Entity
Entity Name
Attributes
Identifier
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Review of Previous Class: Maximum Cardinality
• Relationships are named and classified by their cardinality, which is a word that means count
• maximum cardinality is the maximum number of entity instances that may participate in a relationship instance — one, many or other some fixed number– BASKETBALL-TEAM and PLAYER could be 1:5
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Review of Previous Class: Crow’s Foot Notation
USER VIDEO
USER CHANNEL
EMPLOYEE LOCKER
One Many (crow’s foot)
Attention: do not misplace one and many notations
An employee has (at most) one locker; a locker can belong to (at most) one employee
A user can upload many videos; a video can be uploaded by (at most) one user
A user can subscribe to many channels; a channel can be subscribed by many users
upload
subscribe
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How to Determine Maximum Cardinality?
USER VIDEO
USER CHANNEL
EMPLOYEE LOCKER
Attention: do not misplace one and many notations
An employee has (at most) one locker; a locker can belong to (at most) one employee
A user can upload many videos; a video can be uploaded by (at most) one user
A user can subscribe to many channels; a channel can be subscribed by many users
upload
subscribe
• User requirements• Common Sense
Review Exercise
• Use crow’s foot notation to draw maximum cardinality on the following relationships:
SALESPERSON ORDER
PRODUCT ORDER
EMPLOYEE DEPARTMENT(is manager of)
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This Class
• Entity-Relationship Model– Entity, attribute, identifier, relationship– Maximum Cardinality: 1:1, 1:N, N:M– Minimum Cardinality: Optional, Mandatory
• Case Studies
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Minimum Cardinality
• Minimum cardinality is the minimum number of entity instances that must participate in a relationship instance– typically assumes a value of zero (optional) or one
(mandatory)
Mandatory Optional
Crow’s foot notation
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Minimum Cardinality
EMPLOYEE LOCKERAn employee may have a locker; a locker must belong to an employee
Mandatory Optional
Maximum cardinality notation is closer to entity
Minimum cardinality notation is closer to the middle
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Minimum Cardinality: Exercise
USER VIDEO
Mandatory Optional
What does the Minimum cardinality mean?(A) A user must upload a video; a video may belong to a user(B) A user may upload a video; a video must belong to a user(C) A user may upload a video; a video may belong to a user(D) A user must upload a video; a video must belong to a user
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Minimum Cardinality: Exercise
Which design is most suitable?• User requirements• Common sense
(A)
(B)
(C)
(D)
USER CHANNEL
subscribe
USER CHANNEL
USER CHANNEL
USER CHANNEL
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In-Class Exercise: Minimum Cardinality
In-Class Exercise A-3• Add Minimum Cardinality symbols on the line of the
relationship
In-Class Exercise B-3• Add Minimum Cardinality symbols on the line of the
relationship
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Multiple Relationships between Entities
StudentID ClubIDName
STUDENT CLUB
ClubName
Description
president
member
• Sometimes, there could multiple relationships with different semantic meanings between two entities. • A club will have exactly one student as the president.
Students can also become members of the club.
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Multiple Relationships between Entities
UserID ProductIDName
USER PRODUCT
Name
Description
Sell
Buy
A product must be sold by exactly one user. A product may or may not have a buyer yet.
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Summary
E-R Diagram: Entity-Relationship Diagram(1) entity; (2) identifier; (3) attributes; (4) relationship
UserID Video IDName
Address
USER VIDEO
Title
Description
Length
A complete E-R Diagram represented using Crow’s Foot notation:
USER –> VIDEO: A user may upload zero to many videosVIDEO–> USER: A video must belong to exactly one user