1. Data Modeling using Entity Relationship Diagramming (ERD)

2. ERD Planning Use Cases. DFDs. ERDs. “Working toward an Analysis
actual information
system”.
Analysis
Design
Implementation

3. Use
Cases
Users
DFDs
ERDs
Information System

4. ERD Planning Use Cases. DFDs. ERDs. “Working toward an Analysis
actual information
system”.
Analysis
Design
Physical DFDs and ERDs
go here.
Implementation

5. Items in an ERD
Entity
Relationship
* Attribute (identifier)
Attribute

6. Items in an ERD Entity Relationship Each occurrence of an Entity
* Attribute (identifier)
Attribute
Each occurrence of an Entity
is an “instance”.

7. 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)

8. Cardinality I find it helpful to use this sentence:
A single <instance> <relationship> in at most <either 1 or many> <entity>.

9. 1:1
Dept. Manager
Dept.
contains
Cardinality symbols

10. 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)

11. 1:M
Dept. Manager
Dept. Employee
manages
The “Many” Cardinality symbol

12. 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)

13. M:M
Dept. Projects
Dept. Employee
works on

14. 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

15. Modality I find it helpful to use this sentence:
A single <instance> <relationship> in at least <either 0 or 1> <entity>.

16. 1
Dept. Manager
Dept.
contains
Modality “1” symbols

17. 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

18. Dept. Manager
Dept. Employee
manages
The Modality “0” symbol

19. Both
Dept. Projects
Dept. Employee
works on

20. Steps in Building ERDs Identify the entities
Add appropriate attributes for each entity
Draw the relationships that connect associated entities

21. 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

22. 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!

23. 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.)

24. Level 0 DFD: Registration
Course Offering
Changes 1
Maintain
department
course
offerings
Available
course request
Available
courses
Available
courses
Course Offering
List 2
Maintain
student
enrollments
Students
Available courses
Course enrollment
Course
Offering
Updates
Course
Offerings
Student schedule
Course
enrollment
request D2
Course Offerings
Dept
Staff
Student
schedule
Course
information D3
Enrollments
Fee Payment
History
Student Enrollment
Report Request 3
Course
Enrollment
Reports D1
Fees
Enrollment
information
Student
Enrollment
Report

25. Registration ERD Course Enrollment Offering Dept. # * Student ID
Hours credit
Dept. #
* Course #
Course name
Hours Credit
Max. size
Number
enrolled
includes

26. 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.

27. Level 0 DFD: AREI Sellers Maintain house seller information D2
Sales Contracts
House
Generate
requested
report 1 2
Sales Contract D3
Offered Houses
House information
Buyer information form D1
Multiple Listing
Services File
Sales
Contract
Buyers
House information request D4
Buyer

28. AREI ERD Sales Contract * Seller name Address Phone Listing date
Listing term
Offered
House
Style
Size
Price
Buyer
* Name
Preferred style
Preferred size
Price limit
offers
shown