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

Email

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

Email

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

Email

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

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An online E-R diagram drawing tool: www.draw.io