Transcript Systems Analysis - Northern Illinois University

Data Modeling
using Entity Relationship Diagramming (ERD)
ERD
Planning
Use Cases.
DFDs.
Analysis
Design
Implementation
ERDs.
“Working toward an
actual information
system”.
Users
Use
Cases
DFDs
ERDs
Information System
ERD
Planning
Use Cases.
DFDs.
Analysis
Design
Implementation
ERDs.
“Working toward an
actual information
system”.
Physical DFDs and ERDs
go here.
Items in an ERD
Entity
* Attribute (identifier)
Attribute
Relationship
Items in an ERD
Entity
* Attribute (identifier)
Attribute
Each occurrence of an Entity
is an “instance”.
Relationship
Cardinality

Cardinality refers to the number of times
instances in one entity can be related to
instances in another entity

One instance in an entity refers to one and only one
instance in the related entity (1:1)
Cardinality
I
find it helpful to use this sentence:

A single <instance> <relationship> in at most
<either 1 or many> <entity>.
1:1
Dept. Manager
Dept.
contains
Cardinality symbols
Cardinality

Cardinality refers to the number of times
instances in one entity can be related to
instances in another entity


One instance in an entity refers to one and only one
instance in the related entity (1:1)
One instance in an entity refers to one or more
instances in the related entity (1:M)
1:M
Dept. Manager
Dept. Employee
manages
The “Many” Cardinality symbol
Cardinality

Cardinality refers to the number of times
instances in one entity can be related to
instances in another entity



One instance in an entity refers to one and only one
instance in the related entity (1:1)
One instance in an entity refers to one or more
instances in the related entity (1:M)
One or more instances in an entity refer to one or more
instances in the related entity (M:M)
M:M
Dept. Projects
Dept. Employee
works on
Modality

Modality refers to the minimum number of
times that an instance in one entity can be
related to an instance in another entity

One means that an instance in the related entity must
exist for an instance in another entity to be valid
Modality
I
find it helpful to use this sentence:

A single <instance> <relationship> in at least
<either 0 or 1> <entity>.
1
Dept. Manager
Dept.
contains
Modality “1” symbols
Modality

Modality refers to the minimum number of
times that an instance in one entity can be
related to an instance in another entity


One means that an instance in the related entity must
exist for an instance in another entity to be valid
Zero means that no instance in the related entity is
necessary for an instance in another entity to be valid
0
Dept. Manager
Dept. Employee
manages
The Modality “0” symbol
Both
Dept. Projects
Dept. Employee
works on
Steps in Building ERDs



Identify the entities
Add appropriate attributes for each entity
Draw the relationships that connect
associated entities
ERD Building Tips



Data stores of the DFD should correspond
to entities
Only include entities with more than one
instance of information
Don’t include entities associated with
implementation of the system, not the
system itself
Balancing ERDs with DFDs






All analysis activities are interrelated
Process models contain two data components
 Data flows and data stores
The DFD data components need to balance the ERD’s data
stores (entities) and data elements (attributes)
Many CASE tools provide features to check for imbalance
Check that all data stores and elements correspond between
models
Do not follow thoughtlessly -- check that the models make
sense!
Recall: on-line university registration (from
Use Case & DFD examples)
The system should enable the staff of each academic department to
examine the course offered by their department, add and remove
course, and change the information about them (e.g., the maximum
number of students). It should permit students to examine currently
available courses, add and drop courses to and from their schedules,
and examine the course for which they are enrolled. Department staff
should be able to print a variety of reports about the courses and the
students enrolled in them. They system should ensure that no student
takes too many course and that students who have any unpaid fees
are not permitted to register. (Assume that a fees data store is
maintained by the university's financial office that the registration
system accesses but does not change.)
Level 0 DFD: Registration
Course Offering
Changes
1
Available
courses
Maintain
department
course
offerings
Course Offering
List
D2
Available courses
Course Offerings
Dept
Staff
Course
information
Student Enrollment
Report Request
Student
Enrollment
Report
2
Maintain
student
enrollments
Course
Offering
Updates
Course
Offerings
Available
course request
Course
enrollment
request
D3
Course enrollment
Student schedule
Student
schedule
Fee Payment
History
Enrollments
3
Course
Enrollment
Reports
Enrollment
information
D1
Fees
Students
Registration ERD
Course
Offering
Dept. #
* Course #
Course name
Hours Credit
Max. size
Number
enrolled
Enrollment
includes
* Student ID
Dept #
Course #
Hours credit
Recall: real estate (from Use Case & DFD
examples)
A Real Estate Inc. (AREI) sells houses. People who want to sell their
houses sign a contract with AREI and provide information on their house.
This information is kept in a database by AREI and a subset of this
information is sent to the citywide multiple-listing service used by all real
estate agents. AREI works with two types of potential buyers. Some
buyers have an interest in one specific house. In this case, AREI prints
information from its database, which the real estate agent uses to help
show the house to the buyer (a process beyond the scope of the system
to be modeled). Other buyers seek AREI’s advice in finding a house that
meets their needs. In this case, the buyer completes a buyer information
form that is entered into a buyer database, and AREI real estate agents
use its information to search AREI’s database and the multiple-listing
service for houses that meet their needs. The results of these searches
are printed and used to help the real estate agent show houses to the
buyer.
Level 0 DFD: AREI
Sales
Contract
1
D2
Sales Contract
Sellers
House information
Sales Contracts
Maintain
house
seller
House information
information
House information
D1
Multiple Listing
Services File
D3
Offered Houses
House information
2
House
information
House information request
Buyer information form
Buyers
House information
Generate
requested
report
Buyer
information
D4
Buyers
AREI ERD
Sales
Contract
Offered
House
offers
* Seller name
Address
Phone
Listing date
Listing term
Buyer
shown
* Seller name
Address
Style
Size
Price
* Name
Address
Phone
Preferred style
Preferred size
Price limit