Entity-Relationship Design
Information Level Design
TOP DOWN DATA ANALYSIS
Computer systems are extremely
complicated and cannot be developed
without careful planning. The most
common MIS is the Systems Development
Life Cycle. This approach is to build a
model of the information system based on
the objectives and goals it must meet.
This is called top down modeling.
Babysitter Service
The AITP Service Club wants to run a
babysitting service. Customers call to
request a sitter and the Club
Coordinator assigns an employee to sit
for the customer from a list of
employees available for the particular
day requested.
Entity-Relationship Model
•
A logical representation of the data of
an organization or business area in
graphical form
Enterprise E-R Diagram
Employee
Customer
Job
Data Flow Diagram
Context Diagram
Request
Customer
Confirmation
Babysitter
Information
System
Availability
Assignment
Employee
Data Flow Diagram
Level 1
Request
Assignment
Confirmation
New
1.
Assign
Employee
Current
D1 | Customer
Availability
Avail
Times
Job
Assign
2.
Record
Availability
Avail
Times
D2 | Employee
D3 | Jobs
Communications Model
•
A representation of the location at
which data is stored and processed and
the communications links that connect
them.
Entity Relationship Models
A good E-R model has
One table for every entity in the business
system
 Correctly drawn relationships indicating 1-1
or 1-m cardinalities
 Optionality indicators to support needed
referential integrity

ENTITY:
A person, place, object, event, or
concept about which the
organization wishes to maintain
data.
•
•
•
Must need to store data
Must have at least two attributes
Must have at least two records
ENTITY TYPES
classes of people, objects or concepts
about which we wish to store data.



become tables in a new computer
system.
Instances are rows
Attributes are columns
ATTRIBUTE:
A description or property of a given entity
type.
•
•
•
Must depend on the entity key alone
Must contain information that we
explicitly need
Must have the same data type for all
entity occurrences
RELATIONSHIP:
.
A connection between entity instances in
different entity classes
•
•
Must specify what row connects with
what row in associated tables
Must not describe processing
LOGICAL AND PHYSICAL
COMPONENTS
Logical
Physical
Entity Class
Table
Instance
Row
Attribute
Column
Relationship
Junction Table
or Foreign Key
Primary Key
Identifier
Narrative Description

The conceptual modeling process starts
with a narrative description of the
process. This is a direct, active
depiction of what the system should do.
This is the basis of the initial data and
process models.
Discovering Entities
•
•
•
•
Entities are normally described by NOUNS
and ADJECTIVES
Entities do not change anything.
Entity occurrences are records, entity types
are files.
Reports are derived output and not entities.
Discovering Entities
•
•
•
Entities with only one attribute are usually
modeled as attributes of another entity.
Entities that have only one record are usually
modeled as a set of parameters and not as
files.
Include only files (entity types) that are
needed by a system. Extra entities require
maintenance and space that can add
considerably to the cost of a system.
Converting a text description
into an E-R model:
1. Review the conceptual description of
the business area for nouns that
describe the system.
2. Each entity type should have more
than one potential instance.
3. Each entity type should have more
than one attribute.
4.Each entity type should be relevant..
Relationships
A relationship is a connection between
records in one table and those in
another.


Instructor assigned to class (section)
Student enrolled in class (section)
RELATIONSHIP.
Does not describe processing or change any
data. Relationship names should be passive
(ordered by).
•
•
CARDINALITY Refers to the number of records
that a relationship connects to a given child
record in a relationship.
PARTICIPATION (Optionality) Refers to whether
a record must exist in one table before a related
one is inserted into another.
Diagrams:
1:m Relationships
InstructorID
Section
CourseSection
Instructor
InstructorID
Diagrams:
m:n Relationships
CourseSection
Section
Student-Section
CourseSection
StudentID
StudentID
Student
Optionality
(Referential Integrity)
Records in a table that have a relationship
with another table may be restricted by
optionality requirements.


Relationship Optional
Relationship Mandatory (referential
integrity enforced)
Optionality
Optional (0 allowed)
0
Mandatory (1 or more required)
1
Optionality
A constraint should be mandatory only if
the relationship must be known
whenever a record is first entered.
Most relationships are optional.
Maintaining Integrity
If a parent record is deleted then an
optionality relationships can be
maintained in several ways



Cascade delete
Cascade update
Cascade null
Data