Database Design concepts INFO 1408

Lecture 3

What is a database?

Objectives

Introduce the three layers of data abstractionIntroduce the terms physical and Logical data independence

Towards a definition ………

Take a moment or two to think about how you would define a database. Write down the key words you would use.Describe a database to your neighbour

A typical domestic ‘database’

Is this Organised? Efficient? Easily accessible? Flexible? …… good for much other than keeping things off the floor?

Paper based databases

•Have been around for a long time

•Data is organised and quickly accessibleSo is easier to maintain – fewer errors

•But is not all that flexible – things are filed in one place only (why would it not be a good idea to have duplicates)

A working definition

A collection of data items that are treated as a unit –

Andy Oppel

Are managed by a

We will explore some features of a DBMS ……..Layers of data abstractionPhysical data independenceLogical data independence

Computer based Databases

Some important features of a DBMS

Moves data to and from physical data filesManages multiple usersManages transactions as an ‘all or nothing’ unit of workSupports a Query Language to retrieve data from the databaseManages back upsProvides Security mechanisms

Layers of Data abstraction

Allows us to consider User views of the data – which can be different for each user. More about this later in the term

This is achieved by thinking of 3 Layers of abstraction

Externallayer

Logicallayer

Physicallayer

What the user

How the database the data for viewing

How the computer’s operating system

the data

D

atabaseFile

Databas

eFile

Databas

eFile

Databas

eFile

Databas

eFile

Internal Schema

UserView 1

UserView 2

UserView ….External

layer

Logicallayer

Physicallayer

Physical Data independence

Logical Data independence

The Physical layerThe data files that actually contain the data

Data could be stored over multiple disc drives – by running in parallel

(Microsoft is an exception in that everything is stored in one file – this limits the number of concurrent users, this is why it is not used for large enterprises.)

The DBMS works with the computer’s Operating system (OS) to manage the file storage.

The user of the database does not need to know how the DBMS does this. They just ask for the information and the DBMS supplies it. A little like you ordering a meal at a restaurant, it just arrives- you don’t know how it was cooked or what the ingredients are.

D

atabaseFile

Databas

eFile

Databas

eFile

Databas

eFile

Databas

eFile

Internal Schema

UserView 1

UserView 2

UserView ….External

layer

Logicallayer

Physicallayer

Physical Data independence

Logical Data independence

Logical Layer

The logical layer transforms the physical data into a common This structure is the one that the designer would recognise. It covers all the data requirements of all the users. This is what we will be working on when we start to design databases later on.This ‘abstracted’ format is known as a Depending on the DBMS data could be formatted as 2 dimensional tables or a hierarchical structure (similar to an organisation chart)

D

atabaseFile

Databas

eFile

Databas

eFile

Databas

eFile

Databas

eFile

Internal Schema

UserView 1

UserView 2

UserView ….External

layer

Logicallayer

Physicallayer

Physical Data independence

Logical Data independence

External Layer

This is the layer where users with the database and issue queriesThe DBMS handles the transformation of selected items from the data structures in the logical layer to form each user viewThese user views can be stored for future use, or may be used only temporarily and then discarded – an query.

When we start to create a database we need to look at each user requirements- These requirements will give us all of the views requiredUsing some methods ( that we will learn) we can create the Internal schema which will organise the data so all user requirements can be met.

D

atabaseFile

Databas

eFile

Databas

eFile

Databas

eFile

Databas

eFile

Internal Schema

UserView 1

UserView 2

UserView ….External

layer

Logicallayer

Physicallayer

Physical Data independence

Logical Data independence

Physical Data Independence

There can be differing degrees of independencePrior to systems having data independence any change to the way data was stored would require a change to every computer program that used the data – time consuming and expensive

D

atabaseFile

Databas

eFile

Databas

eFile

Databas

eFile

Databas

eFile

Internal Schema

UserView 1

UserView 2

UserView ….External

layer

Logicallayer

Physicallayer

Physical Data independence

Logical Data independence

Logical Data Independence

The ability to make changes to the logical layer without disrupting existing users and processes

Activities such as: Adding a new database object Adding data items to an existing

object

What key words would you now use to describe a database to a colleague?Take a moment to jot down the key words

Does your definition include words such as..

efficient

organised

flexible

consistent

maintainable

data

accessible

Reliable

Error free

Robust

secure

Summary

We have covered the three levels of abstractionPhysical and logical data independence.

References

Databases Demystified by Andy Oppel

(Published By McGraw-Hill, ISBN 0-07-225364-9)

Data Analysis for database Design by David Howe (published by Arnold Second edition.)