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CS507 - Information Systems - Lecture Handout 25

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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.

Rules for DFD’s

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.

Rules for DFD’s 1

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

Rules for DFD’s 2

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.

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.

Entity Relationship Diagram

Entity Relationship Diagram 1

Entity Relationship Diagram 2

Entity Relationship Diagram 3

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.

Another ERD

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

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

  • 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

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:
    1. Single – as explained above
    2. 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.

Relationships

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

Relationships

Types of Relationship

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

Relationships 1

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

Relationships 2

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

Relationships 3

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.