Rules for DFD’s: Entity Relationship Diagram

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Information System (CS507)

LESSON 25

Rules for DFD's

The DFD as an analytical tool follows certain rules. These rules help in achieving the level of

standardization.

· There should be no black holes, gray holes, miracles.

· Do not leave things unlabeled

· the DFD should not include data flows that are unassociated with the process.

Data Flows cannot be without a process or function being involved in the flow. Below are given three

diagrams that indicate bad practices. There has to be a process in between these three diagrams.

Entity A

Entity B

data store to

entity - or

data store to

entity to

reverse

data store

entity

For a better understanding of data flow diagram, two diagrams are drawn for the same process. The

diagram A includes some errors/ violation of rules usually used and diagram B is the amended picture

how the correct DFD should look like.

The process covered in these diagrams is as follows.

· The number of hours employees have worked is recorded and employee time record is created

stored in employee time file.

· Employee salary information is taken from employee master file which is used along with time

records to calculate gross pay.

· The leads to the calculation of deductions leading to computation of net pay.

· Payrolls slips are printed used for

· Record purpose in the employee master file.

· Preparation of checks

· Paychecks and a copy of pay slip is given to the employees.

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Information System (CS507)

There

must be a

Miracle--No process

process

can have only

between

outputs without an

entity and

input

data store

Avoid Gray Holes process must have

sufficient input to execute

Avoid Black Holes No

process can have only

inputs without an output

The diagram shows the violation of rules along with the relevant explanation. Below we have corrected the

diagram.

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Information System (CS507)

Following corrections have been made:

· A process of creation of employee time record has been added between employee and employee

time file.

· Directions of processes have been corrected among employee master file and processes of gross and

net pay calculation.

· The presentation of payroll printing process has been improved.

25.1 Entity Relationship Diagram

"The entity-relationship diagram (ERD) is a data model or diagram for high-level descriptions of

conceptual data model, and it provides a graphical notation for representing such data models in the

form of entity-relationship diagrams."

E-R Diagram (E-R model) facilitates database design by allowing the specification of an "enterprise

schema" which represents the overall logical structure of a database. The E-R Diagram (E-R model) is

extremely useful in mapping the meanings and interactions of real-world enterprises onto a conceptual

schema.

A company hires many employees, one as a

minimum

COMPANY

HIRES

EMPLOYEES

DEPARTMENT

ASSIGNED TO

RUN BY

WORK ON

PROJECTS

HOD

RUN BY

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Information System (CS507)

·Atleast one employee works on a project.

COMPANY

·An employee may work on more than one

projects at a time.

·At one time, an employee may not be working on

any project.

HIRES

EMPLOYEES

DEPARTMENT

ASSIGNED TO

RUN BY

WORK ON

PROJECTS

HOD

RUN BY

·An employee can be assigned to one

department only.

COMPANY

·One department may have more than one

employees.

HIRES

EMPLOYEES

DEPARTMENT

ASSIGNED TO

RUN BY

WORK ON

PROJECTS

HOD

RUN BY

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Information System (CS507)

·Only one HOD will head the project.

·A HOD of department may head more than one

COMPANY

project.

·At one time, he may not be heading any project.

HIRES

EMPLOYEES

DEPARTMENT

ASSIGNED TO

RUN BY

WORK ON

PROJECTS

HOD

RUN BY

Another ERD

Above we have seen a common example of how a company, its employees, the head of departments,

various projects being worked on are interlinked. ERD can be drawn with a different perspective as well.

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Information System (CS507)

Reservation

Room

Reservation ID

Room ID

User ID

Manager

Date

Building

Start time

Room

End time

Description

Note

Status

User

Session

User ID

Session ID

First

User ID

Last

Session String

Time Stamp

Password

Type

Initials

Status

Log

Log ID

User ID

Time Stamp

In/out

The above figure shows a hotel booking system. Various records have been kept for each entity. However

each entity shares a relationship with for logical purpose. For instance, the field for room ID has been kept

in reservation for access to further data. User information has been kept separate, however link has been

made to reservation, session and logs by making user ID common to all three tables. Such kind of

relationship helps in keeping

25.2 Entity

An entity is an object that exists and is distinguishable from other objects. An entity is described using a set

of attributes. For example specific person, company, event, plant, crop, department, section, cost center.

Entity Set &attributes

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Information System (CS507)

An entity set is a set of entities of the same type that share the same properties

· All entities in an entity set have the same set of attributes, i.e. common characteristics e.g.

names, addresses, date of birth, etc.

· Each entity set has a distinct attribute by which it can be easily identified, e.g. NIC no.,

employee no.

Example

· Bird is an entity

· The class of birds is an entity set

· The color of birds is an attribute

25.3 Value Sets

Each attribute has a Value Set (domain) i.e. defined parameters or the range in which value of the attribute

may fall, e.g.

For Example:

Range of age allowed for employees is between 18 and 60, we can specify the value set of the age attribute

of the EMPLOYEE to be the numbers between 18 and 60.

Attribute Types

There are four types of attributes

· Single / composite :

· Single a single value completely defines the attributes. E.g. The figure 27 represents the age

(attribute) of a person (entity)

· Composite More than one values are required to explain the attribute e.g address includes

house no., street no., postal code, etc for its complete explanation.

Single / multi-valued:

a. Single as explained above

b. Multi-valued Where an attribute can have more than one value. E.g. An individual may

have qualification. This is an attribute. If a person possesses more than one qualification

Null : is a blank read as zero value. E.g. the various categories of graduation degrees (B.A., B.Com.,

BSc, etc) will apply to graduates and not to non-graduates and would be read as "Not Applicable".

Derived : is information provided on the basis of a unique attribute e.g. customer ID, Employee

ID, Student ID. Relevant dependant information can be obtained/derived through the said

attribute.

25.4 Relationships

A relationship is an association among entities. There has to be a relationship between two entities.

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Information System (CS507)

Department

Employees

Account entity

Customer entity

Relationship

Types of Relationship

One to One e.g one supervisor controls one department.

Many to One e.g. More than one employee works in one department

Many to Many More than one employees can work on more than one projects.

Total and partial participation

Total Participation if ALL entities in an entity set "A" are related to the entity set "B". For example, all

employees work for same department or section of the department. Every employee has to be a part of a

department. No employee can exist in an organization without being part of a department.

Partial Participation: if SOME (not all) entities in an entity set "A" are related to the entity set "B". For

example, employees work on various projects undertaken by the organization. It is not essential that each

employee should work on every project.

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