Chapter 55 Data Modelling - Introduction

Compiled by Eddie MoorcroftSource: P M Heathcote A level ICT

Traditional file approach• Most organisations began information

processing on a small scale, by computerising each department independently, one by one.

• Files of information relevant to one department were created and processed by dozens of separate programs.

• This led to many problems:-

Problems with traditional approach:

• Data redundancy– The same data duplicated in many

different files.• Data inconsistency

– Data that is duplicated, and not updated when changed in all files

• Program-data dependence– The computer program requires the data

to be in a particular format, changing the format requires every program using that file to be changed.

Problems with traditional approach - continued• Lack of flexibility

– In such a system, when information of a non-routine nature is needed, it may take weeks to assemble the data from various files.

• Data not shareable– If one department has data that was

required by another, it can be awkward to obtain it.

The database approachThe database approach• A database is defined as:

“a collection on non-redundant data shareable between different applications”

• Thus, all data belonging to the entire organisation would be centralised in a common pool of data, accessible by all applications.

Problems with the Database approach• Unproductive maintenance

– Programs still depend on the structure of data files, so if one department required to add an extra field, all programs would need to be altered.

• Lack of security– All the data in the database, even

confidential or sensitive data, is accessible by all applications.

The Database Management System (DBMS)• This is a layer of software inserted

between the applications and the data, which attempts to solve the problems of “Unproductive maintenance” and “Lack of Security”

• Two features of DBMS are:– Program-data

independence– Restricted user access

Diagram of DBMS System

CompanyDatabase

DatabaseManage-

mentSystem

Salesprograms

Purchasingprograms

Financeprograms

Payrollprograms

Personnelprograms

Conceptual data model• A database is designed based on the

information it will hold. The requirements are defined in terms of:

• An entity– a thing of interest to an

organisation, about which the data is to be held – e.g. customer, employee, stock item, etc.

Conceptual data model - continued

• An attribute– a property or characteristic of an

entity – e.g. attribute associated with a student are, surname, forename, date of birth, admission number etc.

• A relationship– the links or association between

two entities, e.g. between school and pupil, a school contains many pupils, but a pupil only goes to one school.

Joe Bloggs,25/03/83,012345

Entity-relationship diagrams

Employee Company cardrives

(One to One)

Ward Patientsholds(One to Many)

CD Album Singersfeatures

(Many to Many)