Data Modeling using Entity Relationship Diagramming (ERD)

1
Data Modelingusing Entity Relationship
Diagramming (ERD)
2
ERD
Planning
Use Cases. DFDs. ERDs. Working toward
an actual information system.
Analysis
Design
Implementation
3
Use Cases
Users
DFDs
ERDs
Information System
4
ERD
Planning
Use Cases. DFDs. ERDs. Working toward
an 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
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 (11)

8
Cardinality

• I find it helpful to use this sentence
• A single ltinstancegt ltrelationshipgt in at most
  lteither 1 or manygt ltentitygt.

9
11
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 (11)
• One instance in an entity refers to one or more
  instances in the related entity (1M)

11
1M
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 (11)
• One instance in an entity refers to one or more
  instances in the related entity (1M)
• One or more instances in an entity refer to one
  or more instances in the related entity (MM)

13
MM
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 ltinstancegt ltrelationshipgt in at least
  lteither 0 or 1gt ltentitygt.

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
0
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
• Dont 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 ERDs
  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
Available course request
Available courses
Available courses
Course Offering List
Students
Available courses
Course enrollment
Course Offering Updates
Course Offerings
Student schedule
Course enrollment request
Dept Staff
Student schedule
Course information
Fee Payment History
Student Enrollment Report Request
Enrollment information
Student Enrollment Report
25
Registration ERD
Course Offering
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 AREIs 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 AREIs 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
28
AREI ERD