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Data Modeling
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Data Modeling
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Process of creating a logical
representation of the structure of the
database
The most important task in database
development
E-R Model
Semantic Object Model
Relational Model
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Entity Relationship Data Model
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Entity-Relationship Data Model
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Explicitly represents entities and their
relationships
Implementation independent
For use at the conceptual and external
levels
Peter Pin-San Chen, The entity-relationship model---toward a
unified view of data. ACM Transactions on Database Systems,
Vol.1, No. 1, March 1976 - portal.acm.org
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A picture with Peter Chen, the founder of E-R model
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Entity-Relationship
Data Model
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Entity
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Attribute
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Characteristic or property of an entity
Relationship
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A thing, event, or person in the organization's
environment about which someone wants to collect data
Association between entities
Composite entity (Associative entity)
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Represents both an entity and relationship
Has its own properties
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Components of E-R Model
Entity
Relationship
Composite entity
Attribute
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An E-R Model for
Student Registration System
Entities
Course
Instructor
Student
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An E-R Model for
Student Registration System
Relationships
One-to-many
Course
Teaches
M
Instructor
1
N
1
Advises
M
Course Enrollment
Student
M
Many-to-many
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An E-R Model for
Student Registration System
Course
Teaches
M
Instructor
1
1
1
Advises
M
M
Student
Class Enrollment
1
M
Composite
Entity
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An E-R Model for
Student Registration System
Course
Number
Instructor ID
Description
Room
Course
Attributes
Rank
Teaches
M
Name
Instructor
1
1
1
Advises
M
M
Student
Class Enrollment
M
Course
Number
Grade
Student
Number
1
Student
Number
Major
Student
Name
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Entities
“something that users track”
Page 49
Figure 3-1
© 2000 Prentice Hall
Attributes (properties)
“describe the entity’s characteristics”
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Entity:Employee
Attributes:EmployeeName, Extension,
DateOfHire, JobSkillCode
Attribute type
Key attribute or identifier (i.e. employee number)
 Single value attribute (i.e. spouse)
 Multivalued attribute (i.e. children)
 Derived attribute (i.e. tax payment)
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More about Entities
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Existence Dependency
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The existence of one entity depends on the
existence of one or more other entities
Fundamental entity vs. Attributive entity or
Weak entity
A weak entity is one that is existence-dependent
and has a primary key that is partially or totally
derived from the parent entity in the relationship.
Entity Supertypes and Subtypes
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Exclusive (disjoint) and Inclusive (overlapping)
Covering and non-covering
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Subtype Entities
“an entity that contains option sets of
attributes”
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Figure 3-10b
© 2000 Prentice Hall
Relationships
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A relationship is an association between
entities.
A unary relationship exists when an
association is maintained within a single
entity. (Recursive relationship)
A binary relationship exists when two entities
are associated. (Most common)
A ternary relationship exists when three
entities are associated. (e.g. real estate
sales: buyer, seller, and property)
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Classroom Assignment
Course
1
M
Classroom
Assignment
Classroom
M
1
M
1
Time Slots
Convert a ternary
relationship to three
binary relationships
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Relationships
“associations between entities”
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Figure 3-3
© 2000 Prentice Hall
Connectivity and Cardinality
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The term connectivity is used to describe the
relationship classification (e.g., one-to-one,
one-to-many, and many-to-many).
Cardinality expresses the specific number of
entity occurrences associated with one
occurrence of the related entity.
Relational Participation: Optional, Mandatory
The actual number of associated entities is
usually a function of an organization’s policy.
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Connectivity and Cardinality
Connectivity
Professor
1
M
teaches
(0,3)
Course
(1,1)
Optional entity
Mandatory entity
Cardinality
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Building E-R Model
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Understand business rules and
assumptions
Identify entities
Identify relationships
Identify attributes
Check and improve E-R model
Convert E-R model to relational tables
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Business rules
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A business rule is a statement that
defines or constrains some aspect of
the business.
Business rules and policies govern
creating, updating, and removing data
in an information processing and
storage system.
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Example of business rules
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A student can take up to five courses
each term
Each course will be taught by only one
teacher
A customer must use valid credit card
to make online payment
A flower order can deliver to anywhere
in North America but only one location
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Business rules
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As a database analyst, you need to:
Identify and understand those rules
that govern data
Represent those rules so that they can
be unambiguously understood
Implement those rules in database
technology
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Sales Order Form
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An E-R Model for Order
Processing:
Order
Number
Date
S ID
S Name
Product
Number
Customer
Name
Orders
Description
Address
Unit price
Quantity
Total
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An E-R Model for Order
Processing: Entities
Customer
Products
Salesman
Orders
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An E-R Model for Order
Processing: Relationships
Customer
Products
Salesman
1
1
1
Prepared
Placed
M
OrderLine
M
Orders
M
1
M
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An E-R Model for Order
Processing: Attributes
Customer Name
Description
Unit price
S Name
S ID
Address
Customer
Product
Number
Products
Salesman
1
1
1
Prepared
Placed
M
OrderLine
M
Orders
M
1
M
S ID
Product
Number
Quantity
Order
Number
Order
Number
Customer
Name
Date
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Crow's Foot Notation of E-R Diagram
with Attributes Shown
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Cardinality Symbols of
Relationships for ERD
Check and improve E-R model
Convert a multivalued attribute into an
attribute entity or weak entity
 Introduce composite entity to convert a
m:n relation into two 1:m relations.
Introduce a composite key
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Check and improve E-R model
Each entity must have a key (simple or
composite)
 Make each attribute associate with only
one entity unless it is a foreign key
 Make each entity represent a simple object
or concept
 Divide complex entity into several related
simple entities
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Example:A Construction
Company
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A manager may manage many projects.
Each project requires the services of many employees.
An employee may be assigned to several different
projects.
Some employees are not assigned to a project and
perform duties not specifically related to a project.
Some employees are part of a labor pool, to be shared
by all project teams.
Each employee has a (single) primary job
classification. This job classification determines the
hourly billing rate.
Many employees can have the same job classification.
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A E-R Model for A
Construction Company
Project
Employee
Assignment
Job class
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A E-R Model for A
Construction Company
Subclass
Manager
Project
Worker
Employee
Assignment
Job class
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A E-R Model for A
Construction Company
Project
M
1
Manages
1
1
M
Assignment
Employee
M
Has
M
1
Job class
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A E-R Model for A
Construction Company
Project
Number
Project
name
Project
Manager
ID
M
1
Manages
1
Employee
1
M
Assignment
Employee
Name
Employee
Number
Hire
Date
M
Has
M
1
Job class
Assignment
Number
Hour
Rate
Hours
Project
Number
Employee
Number
Job
Code
Job
Description
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The Semantic Object Model
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Semantic Object Model
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E. F. Codd, “Extending the Relational
Model to Capture More Meaning,” ACM
Transactions on Database Systems,
Dec. 1976, pp. 397-424.
Micheal Hammer and Dennis McLeod,
“Database Description with SDM: A
Semantic Database Model,” ACM
Transactions on Database Systems,
Setp. 1981, pp 351-386.
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Semantic Objects
“a named collection of attributes that
sufficiently describes a distinct entity”
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Student
Customer
Employee
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Attributes
“define the characteristics of semantic objects”
Page 75
Figure 4-2a
© 2000 Prentice Hall
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Attribute Cardinality
“indicates the minimum or maximum number of
instances of the attribute that must exist in
order for the object to be valid”
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Figure 4-2b
© 2000 Prentice Hall
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Object Instances
“the occurrence of a particular semantic object”
Page 77
Figure 4-3
© 2000 Prentice Hall
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Object Identifier
“one or more object attributes that the
users employ to identify object
instances”
Example: CustomerID for Customer
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Domain
“a description of an attribute’s possible
values”
Page 78
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Semantic Object View
“the portion of an object that is visible to a
particular application; view”
Page 79
Figure 4-4
© 2000 Prentice Hall
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Highline University
Administration Database
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Objects:
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College
Department
Professor
Student
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Semantic Object Diagrams
Page 85
Figure 4-13 (1)
© 2000 Prentice Hall
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Semantic Object Diagrams
Page 85
Figure 4-13 (2)
© 2000 Prentice Hall
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Semantic Object Diagrams
Page 85
Figure 4-13 (3)
© 2000 Prentice Hall
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Semantic Object Diagrams
Page 85
Figure 4-13 (4)
© 2000 Prentice Hall
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Sales Order Form
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Semantic vs. E-R Model
Page 105
Figure 4-34
© 2000 Prentice Hall
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Semantic vs. E-R Model
Page 105
Figure 4-33
© 2000 Prentice Hall
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