Database Design
Dr. M.E. Fayad, Professor
Computer Engineering Department, Room #283I
College of Engineering
San José State University
One Washington Square
San José, CA 95192-0180
http://www.engr.sjsu.edu/~fayad
2003
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ER Diagrams
Lesson 2:
Data Modeling Using
Entity-Relationship
Model
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Lesson Objectives
Objectives
Understand the notation of ERDs
 Learn how to do ER Analysis
 Illustrate examples for ERDs

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What Is Entity-Relationship Diagrams?
 E-R
diagrams (ERDs):
– Identify major ingredients, entities, of a
complicated situation
– Identify characteristics, attributes, of the
major ingredients
– Uncover and analyze interconnections,
relationships, among the major ingredients
– Assess various aspects of the
relationships
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What Is Entity-Relationship Diagrams?


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ERD – Chen 1976
Precursor to the class diagram – Will be covered
later.
No generalization
ERD
– Entities: Information holding structures
– Relationships: Connections between entities
– Cardinalities: Upper bounds
– Modalities: Lower bounds
– Attributes: Properties, characteristics, …
associated with an entity.
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What Is Entity-Relationship Diagrams?
 In
short, ERD strives
– to take a close look at
the ingredients of a
puzzle
– in order to identify
major and minor
ingredients
– and to figure out how
they relate to each
other
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Entity-Relationship Diagrams
 ERD
can be summarized as the
process of uncovering, analyzing, and
explicating (i.e., stating clearly and fully)
the business rules governing a
complicated situation
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Business Rules (1)

Business rules are precise statements,
derived from a detailed description of the
organization's operations, that define one or
more of the following modeling components:
–
–
–
–
–
–
Entities
Relationships
Attributes
Connectivities
Cardinalities
Constraints
8
P.; Coronel, C. Instructor’s Manual to Accompany Database Systems:
Design, Implementation, and Management. Fourth Edition. CD-ROM
1Rob,
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Business Rules (2)

Examples of business rules1
– An invoice contains one or more invoice lines, but each
invoice line is associated with a single invoice. That is,
• An invoice contains one or more invoice lines.
• Each invoice line is associated with a single invoice.
– A store employs many employees, but each employee is
employed by only one store.
– A college has many departments, but each department
belongs to a single college. (This business rule reflects a
university that has multiple colleges such as Business,
Liberal Arts, Education, Engineering, etc.)
Rob, P.; Coronel, C. Instructor’s Manual to Accompany Database Systems:
Design, Implementation, and Management. Fourth Edition. CD-ROM
1From:
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Business Rules (3)

Examples of business rules1
– A driver may be assigned to drive many different
vehicles, and each vehicle can be driven by many
drivers. (Note: Keep in mind that this business rule
reflects the assignment of drivers over a period of
time.)
– A client may sign many contracts, but each
contract is signed by only one client.
– A sales representative may write many contracts,
but each contract is written by one sales
representative
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Business Rules (4)
 Some
business rules for a library
– Books and other materials exist in various
editions and numbers of copies
– Books may or may not circulate
– Books that circulate may do so for varying
periods
• Periods may be determined by
– Book status alone
– Borrower status alone
– Combination of book status and borrower status
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Business Rules (5)
 More
business rules for a library
• Users may have various statuses
• Users may have varying numbers of
items currently charged out to them
• Users may or may not owe fines
– Fine status may or may not affect eligibility
to borrow
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Business Rules (6)

More business rules for a library
• Staff members have attributes of
– Salary or wage rates
– Annual leave
– Sick leave
– Work schedules
– Job titles
– Special skills (e.g., languages, puppetry)
– Hire dates
– Total length of service
– Professional organization memberships
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Business Rules (7)
 More
business rules for a library
• Physical facilities include
– Branches
– Furniture
– Machines (e.g., computers, microform readers,
automobiles, players for CD-ROMs, audiotapes,
videotapes)
– Library-management software
– Catalog records (e.g., computerized databases,
catalog cards)
– Records of inventory, acquisitions, circulation,
personnel
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Business Rules (8)
 More
business rules for a library
• Related organizations may include
– City, university, or corporate administrations
– Employee unions
– Friends of the Library organizations
– Vendors of books, serials, and other products
and services used by the library
– Networks of which the library is a member (e.g.,
OCLC, Amigos, Association of Research
Libraries, Texas State Library network, Central
Texas Library System)
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E-R Diagrams (1)

E-R analysis relies heavily
on diagrams to represent
–
–
–
–
Entities (rectangles)
Attributes (ellipses)
Relationships (diamonds)
Connections (lines, which may
have arrowheads or crowsfeet
to represent degrees of
connectivity or relationship)
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Entity Relationship Diagrams
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Data models are tools used in analysis to describe
the data requirements and assumptions in the
system from a top-down perspective. There are
three basic elements in ER models:
– Entities are the "things" about which we seek
information.
– Attributes are the data we collect about the
entities.
– Relationships provide the structure needed to
draw information from multiple entities.
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Developing ERDs
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Developing an ERD requires an understanding of
the system and its components. Before discussing
the procedure, let's look at a narrative created by
Professor Harman.
 Consider a hospital: Patients are treated in a
single ward by the doctors assigned to them.
Usually each patient will be assigned a single
doctor, but in rare cases they will have two.
 Heathcare assistants also attend to the
patients, a number of these are associated with
each ward.
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Developing ERDs
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Initially the system will be concerned solely with
drug treatment. Each patient is required to take a
variety of drugs a certain number of times per day
and for varying lengths of time.
The system must record details concerning patient
treatment and staff payment. Some staff are paid
part time and doctors and care assistants work
varying amounts of overtime at varying rates
(subject to grade).
The system will also need to track what treatments
are required for which patients and when and it
should be capable of calculating the cost of
treatment per week for each patient (though it is
currently unclear to what use this information will be
put).
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How do we start an ERD? (1)
1. Define Entities: these are usually nouns used in
descriptions of the system, in the discussion of
business rules, or in documentation; identified in
the narrative (see highlighted items above).
2. Define Relationships: these are usually verbs used
in descriptions of the system or in discussion of
the business rules (entity ______ entity); identified
in the narrative (see highlighted items above).
3. Add attributes to the relations; these are
determined by the queries,and may also suggest
new entities, e.g. grade; or they may suggest the
need for keys or identifiers.
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How do we start an ERD? (2)
What questions can we ask?
a. Which doctors work in which wards?
b. How much will be spent in a ward in a given
week?
c. How much will a patient cost to treat?
d. How much does a doctor cost per week?
e. Which assistants can a patient expect to see?
f. Which drugs are being used?
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How do we start an ERD? (3)
4. Add cardinality to the relations
Many-to-Many must be resolved to two one-tomanys with an additional entity
Usually automatically happens
Sometimes involves introduction of a link entity (which
will be all foreign key) Examples: Patient-Drug
5. This flexibility allows us to consider a variety
of questions such as: a. Which beds are free?
b. Which assistants work for Dr. X?
c. What is the least expensive prescription?
d. How many doctors are there in the hospital?
e. Which patients are family related?
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6. Represent that information with symbols.
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ERDs

Overview
 Concepts
 Rules
 Guidelines
<See Documents with Lecture 04>
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Crucial Use of E-R Analysis (1)
 It
is especially important to identify and
note many-to-many (M:N) relationships:
for example1
– Note: The small circle denotes optionality (a person
need not be a customer; but to be a customer, he or
she must rent at least one tape)
M
CUSTOMER
1From:
N
rents
TAPE
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Rob, P.; Coronel, C. Instructor’s Manual to Accompany Database Systems:
Design, Implementation, and Management. Fourth Edition. CD-ROM
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ER Diagrams
Crucial Use of E-R Analysis (2)
 The
typical resolution of many-to-many
(M:N) relationships is by a bridge table
(also called a composite entity)
1
M
CUSTOMER
1
M
TAPE
RENTAL
(1,N)
(1,1)
(1,1)
(0,N)
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Crucial Use of E-R Analysis (3)
 A further
explication of the resolution of
the M:N relationship
1
M
generates
CUSTOMER
(1,N)
M
enters
RENTAL
(1,1)
1
(1,1)
TAPE
(0,N)
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Discussion Questions
T/F
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1.
ERD supports generalization.
2.
Cardinality of one is symbolized by a bar -- | --written
across the relationship line.
3.
An entity type must have at least one relevant attribute
type.
4.
Entity Type Name is Synonyms and Homonyms.
5.
Cardinality: A business rule indicating the number of
times a particular object or activity may occur.
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Tasks for Next Lecture
Task 1: Team forming – Team name,
Team members’ names (2-3) per
team, and team members’ e-mails
Task 2: Contact information – Email
your name, e-mail, and phone #
Task 3: Practical exercise: ERDs
Task 4: Assignment #1
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