Download Chapter 3: ER Model

Data Modeling
Yong Choi
School of Business
CSUB
Part # 2
Database

Collection of data in electronic format
–
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A digital library of organization
Managed by one set of software that
provides access to all the data
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No data redundancy, data inconsistency,
poor security, application-data
dependency...
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Database Management
System (DBMS)
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Database software
Act as an interface between application
and physical data files
Support centralization of data
Independent of specific computer
programs
small (MS Access), large/popular
(Oracle)
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Database Models
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Hierarchical (Tree) Models
Network Model
Relational Models
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Relational Database Model
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In the relational data model the database is
represented as a group of related tables.
The relational data model was introduced in
1970 by E. F. Codd of IBM published a paper
in CACM entitled "A Relational Model of Data
for Large Shared Data Banks".
It is currently the most popular model. The
mathematical simplicity and ease of
visualization of the relational data model have
contributed to its success.
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Relational Database
A schematic diagram of a relational database (a) and a sample part of a
relational database showing different tables (b)
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Relational database and keys
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A relational database is a collection of tables
that are related to one another based on a
common field.
A field, or a collection of fields, is designated
as the primary key.
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The primary key uniquely identifies a record in the
table.
When the primary key of one table is
represented in a second table to form a
relationship, it is called a foreign key.
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Primary Key
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Unique identifier
–
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Prevent confusion
Cost of PK
–
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Last name vs. SS#
SS# vs. finger print
Entity Integrity Rule
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Any primary key is allowed to accept null
values.
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Foreign Key
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An attribute in one table whose values
must either match the primary key in
another table or be null.
The database must not contain any
unmatched foreign key values.
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Primary Key
Foreign Keys
Part # 2
Student
ID
Last
Name
First
Name
Dor
m
2144
Arnold
Betty
3122
Taylor
John
Smit
h
Jone
s
3843
Simmons
Lisa
Primary Key
9844
Macy
Bill Key
Foreign
from
2837
Leath Duplicated
Heather
Dorm Residence
primary key of
Director
Major entity
2293
Wrench (notTim
unique)
Smith Andrea
Fernandez
Jones Daniel Abidjan
Smit
h
Smit
h
Jone
s 10
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Foreign Keys
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Relating tables using a common
field
Part # 2
The primary key in the
Employer table (EmployerID) is
the common field that relates
this table to the Position table.
PositionID is the primary key in
the Position table. The EmployerID
field is a foreign key in this table.
Primary keys can only have one
occurrence in a table. Foreign keys
may have multiple occurrences.
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Why Conceptual Data
Modeling is Important?
Part # 2
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Effective Communication Tool
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User involvement
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Independence from a particular DBMS
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Documentation
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Entity Relationship diagram (ERD)
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ERD is commonly used to:
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Translate different views of data among
managers, users, and programmers to fit
into a common framework.
Define data processing and constraint
requirements to help us meet the different
views.
Help implement the database.
Developed by Peter Chen (1976).
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ERD using Chen’ Notation
Part # 2
Part # 2
ER model using IE Notation
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Basic Entity Relationship Diagram
(ERD) Elements
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Entity : a collection of people, places, objects,
events, concepts of interest (a table)
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Entity instance – a member of the Entity : a person,
a place, an object … (a row in a table)
Attribute - property or characteristic of interest
of an entity (a field in a table)
Relationship – association between entities
(corresponds to primary key-foreign key
equivalencies in related tables)
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Entity
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What should be an Entity:
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An object that will have many instances in the
database
An object that will be composed of multiple
attributes
An object that we are trying to model
What should NOT be an Entity:
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A user of the database system
An output of the database system (e.g. a report)
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Part # 2
Entity …
Inappropriate entities
System user
System output
Appropriate entities
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Entity Instance
Entity instance: a single occurrence of an entity.
entity
instance
Student Last
ID
Name
First
Name
2144
Arnold
Betty
3122
Taylor
John
3843
Simmons Lisa
9844
Macy
Bill
2837
Leath
Heather
2293
Wrench
Tim
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Attributes
“describe the entity’s characteristics”
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Attributes are represented by an
Ellipse/Oval connected to the entity with a
line by Chen’s notation.
Each oval contains the name of the
attribute it represents.
Entity:
Attributes:
Employee
Employee-Name, Extension, Date-Of-Hire,
Job-Skill-Code
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ERD using Chen’ Notation
Part # 2
Part # 2
Classes of attributes
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Simple Attribute
Composite Attribute
Derived Attributes
Single-valued
Multi-valued Attribute
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How to find entities and attributes??
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Entity: a collection of people, places, objects,
events, concepts of interest
 look for nouns
 Usually, a proper noun is not a good
candidate….
Attribute: property or characteristic of interest
of an entity
 look for attribute descriptive words
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Part # 2
(unique) Identifier
“attributes that uniquely identify entity instances”

Uniquely identify every instance of the entity

Composite identifiers are identifiers that consist of
two or more attributes
Entity:
Employee
Identifier: Social-Security-Number
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Part # 2
Entities???
ANG Laboratory has several chemists who work on one or more projects.
Chemists also may use certain kinds of equipment on each project. The
organization would like to store the chemist’s employee identification
number, his/her name, up to three phone numbers, his/her project
identification number and the date on which the project started. Every piece
of equipment, the chemist uses, has a serial number and a cost. Also the
organization would like to store the date the chemist was assigned to the
project and the date an equipment item was assigned to a particular chemist
working on a particular project. A chemist must be assigned at least to one
project and one equipment item. Any given equipment item need not be
assigned, and a given project need not be assigned either a chemist or an
equipment item.
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Part # 2
Entities
Project
Chemist
Equipment
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Attributes???
ANG Laboratory has several chemists who work on one or more
projects. Chemists also may use certain kinds of equipment on each
project. The organization would like to store the chemist’s employee
identification number, his/her name, up to three phone numbers, his/her
project identification number and the date on which the project started.
Every piece of equipment, the chemist uses, has a serial number and a
cost. Also the organization would like to store the date the chemist was
assigned to the project and the date an equipment item was assigned to
a particular chemist working on a particular project. A chemist must be
assigned at least to one project and one equipment item. Any given
equipment item need not be assigned, and a given project need not be
assigned either a chemist or an equipment item.
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Part # 2
Entities & Attributes
Emp#
Phone#
Chemist
Equipment
Proj#
Start-Date
Project
Serial#
cost
How many attributes do you (as a entity of our school
database) have? – name your attributes
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Part # 2
Example 1: find entities
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A department hires many employee. A
employee is employed by one department.
A manager manages one department. A
department is managed by one manager.
A person must have only one DNA pattern
and that pattern must be applied to only one
person.
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Part # 2
Example 2: find entities

There are several departments in School of
Business and Public Administration at CSUB.
Each department belongs to one school. A
department has many professors, but a
professor may belong to more than one
department. Similarly, courses may be offered
by several departments and a department
must have at least one course. A professor
may teach more than one student, and a
student may have more than one professor.
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Part # 2
Example 4: Find all entities, attributes,
and (unique) identifier
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The FANTASTIC video rental company has several
branches throughout the USA. The data held on each
branch is the branch address made up of street, city,
state, and zip code, and the telephone number. Each
branch is given a branch number, which is unique
throughout the company. Each branch is allocated staff,
which includes a Manager. The Manager is responsible for
the day-today running of a given branch. The data held on
a member of staff is his or her name, position, and salary.
Each member of staff is given a staff number, which is
unique throughout the company. Each branch has a stock
of videos. The data held on a video is the catalog number,
video number, title, category, daily rental, cost, status,
and the names of the main actors, and the director. The
catalog number uniquely identifies each video.
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Part # 2
Example 4: Find all entities, attributes,
and (unique) identifier (con’t)
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A customer must first register as a member of a local
branch before renting a video tape from any branches. The
data held on a member is the first and last name, address,
and the date that the member registered at a branch. Each
member is given a member number, which is unique
throughout all branches of the company. Once registered,
a member is free to rent videos, up to maximum of ten at
any one time. The data held on each video rented is the
rental number, the full name and number of the member,
the video number, title, and daily rental, and the dates the
video is rented out and date returned. The rental number
is unique throughout the company.
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Relationships
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Association between entities (by lines)
Any connected entities are called participants
Operate in both directions
 Relationship between Student and Curriculum
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A student is enrolled in many curriculums.
Each curriculum is being studied by many students.
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Part # 2
Relationships ???
ANG Laboratory has several chemists who work on one or more
projects. Chemists also may use certain kinds of equipment on each
project. The organization would like to store the chemist’s employee
identification number, his/her name, up to three phone numbers, his/her
project identification number and the date on which the project started.
Every piece of equipment, the chemist uses, has a serial number and a
cost. Also the organization would like to store the date the chemist was
assigned to the project and the date an equipment item was assigned to
a particular chemist working on a particular project. A chemist must be
assigned at least to one project and one equipment item. Any given
equipment item need not be assigned, and a given project need not be
assigned either a chemist or an equipment item.
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Part # 2
Entities/Relationships
& their Attributes
Start-Date
Proj#
Project
Works-On
Emp#
Phone#
Date-Assigned
Chemist
Equipment
Uses
Serial#
cost
Assign-Date
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Part # 2
Steps for creating an ERD
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Develop draft business rules
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Identify the entities (look for nouns)
Identify the attributes (look for attribute descriptive
words)
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identify draft business rules
Identify the relationships (look for verb or verb phrases
between entities)
Identify cardinalities (look for participation related
words and phrases such as zero, none, a, one,
several, many…..)


Using interviews and documents
Optional (look for auxiliary verbs such as may, might, can and
based upon own judgment..)
Set business rules
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Part # 2
Cardinality of Relationship
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One – to – One (1:1)
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One – to – Many (1:M)
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Each instance in the relationship will have exactly
one related member on the other side
A instance on one side of the relationship can
have many related members on the other side,
but a member on the other side will have a
maximum of one related instance
Many – to – Many (M:N)

Instances on both sides of the relationship can
have many related instances on the other side
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Part # 2
1:1 relationship in set notation
DEPARTMT
EMPLOYEE
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Part # 2
Example of 1:1 relationship
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A one-to-one (1:1) relationship is when at most
one instance of a entity A is associated with one
instance of entity B.
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Each employee has a one’s own office.
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Cardinality toward the Office entity
Each office is occupied by one employee.
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Cardinality toward the Employee entity
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Part # 2
1:1 relationship
A person must have one and only one DNA
pattern and that pattern must apply to one and
only one person.
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Part # 2
1:M relationship in set notation
DEPARTMT
EMPLOYEE
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Part # 2
Example of 1:M relationship

A one-to-many (1:N) relationships is when for
one instance of entity A, there are zero, one, or
many instances of entity B, but for one
instance of entity B, there is only one instance
of entity A.
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Each employee has many offices.
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Cardinality toward the Office entity
Each office is occupied by one employee.
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Cardinality toward the Employee entity
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Part # 2
1:M relationship
Each department hires many employees, and
each employee is hired by one department.
HIRES
DEPARTMT
IS_HIRED_BY
EMPLOYEE
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Part # 2
M:N relationship in set notation
WAREHOUSE
PRODUCT
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Part # 2
Example of M:N relationship

A many-to-many (M:N) relationship, sometimes called
non-specific, is when for one instance of entity A,
there are zero, one, or many instances of entity B
and for one instance of entity B there are zero, one,
or many instances of entity A.
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A employee has multiple offices.
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Cardinality toward the Office entity
A office must be occupied by many employee.
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Cardinality toward the Employee entity
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Part # 2
M:N relationship
Each student takes many classes, and each
class is taken by many students.
IS_TAKEN_BY
STUDENT
CLASS
TAKE
** This is not a desirable relationship.**
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Part # 2
Complete ER Diagram
Start-Date
Proj#
1
Emp#
Works-On
N
Project
Phone#
Date-Assigned
Chemist
1
Uses
N
Equipment
Serial#
cost
Assign-Date
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Cardinality Notations
Cardinality
Interpretation
Minimum
Instances
Maximum
Instances
Exactly one
1
Zero or one
0
One or many
1
Many (>1)
Zero or many
0
Many (>1)
Part # 2
Graphic
Notation
1
1
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Part # 2
Steps for creating an ERD

Develop draft business rules



Identify the entities (look for nouns)
Identify the attributes (look for attribute descriptive
words)



identify draft business rules
Identify the relationships (look for verb or verb phrases
between entities)
Identify cardinalities (look for participation related
words and phrases such as zero, none, a, one,
several, many…..)


Using interviews and documents
Optional (look for auxiliary verbs such as may, might, can and
based upon own judgment..)
Set business rules
50