Unit 4Unit 4

NormalisationNormalisation

andand

Relational Database Relational Database

Management SystemsManagement Systems

Computerised Databases

A database is a organised collection of data items and links between them structured in such a way that it allows it to be accessed by a number of different application programs.

i.e. related tables of data which are linked together and many programs can use the data in the tables

Use a database when you need to store lots of data

Members No

Forename Surname Address 1 Address 2 Tel No Video No

122 Evans Daniel 7 High St Aberaeron 01545588999 343

759 Lees Anna 2 Bridge St

Lampeter 01570422967 523

122 Evans Daniel 7 High St Aberaeron 01545588999 455

Title Certificate Category Cost per day

Date loaned

Length of loan

Date due

Total cost

Crash 12 Comedy £3.00 12/09/05 1 day 13/09/05 £3.00

Emma 12 Drama £3.00 12/09/05 1 day 13/09/05 £3.00

War of the Worlds

PG Sci FI £3.59 14/09/05 2 days 16/09/05 £3.50

Duplication in Flatfile

Members Member number

Forename

Surname

Address line 1

Address line 2

Telephone number

Videos Video number

Title

Certificate

Category

Cost per day

Loans Member number

Video number

Date loaned

Length of loan

Date due

Total cost

Linked data tables in a relational database

Primary KeyForeign Key

Primary Key

Loans ID Primary Key

NormalisationNormalisation

Terminology: Primary KeyTerminology: Primary Key

A primary key is a A primary key is a unique valueunique value which allows which allows each record to be identifiedeach record to be identified

CustomerID FirstName LastName

1 Brian Smith

2 Harry Adams

3 Joe Jones

4 Harry Smith

FirstName or LastName cannot be primary keys as they contain duplicate and un-unique data. CustomerID uniquely identifies a row and is therefore suitable

Sometimes there is no single field appropriate Sometimes there is no single field appropriate as a primary key. In these circumstances, it as a primary key. In these circumstances, it is possible to select two fields which, when is possible to select two fields which, when taken together create a unique value:taken together create a unique value:

OrderNo ItemNo EmployeeNo CustomerNo ItemName Quantity

121 3 4 1024 Nut 4

121 4 4 1024 Bolt 3

122 8 9 176 Washer 6

123 3 6 154 Bolt 5

123 8 6 154 Washer 4

There are no unique fields, so the Primary Key is best suited by OrderNo and ItemNo taken together

Foreign KeyForeign Key

A Foreign key is a primary key of another A Foreign key is a primary key of another table used to link the tablestable used to link the tables

Second Normal Form (2NF)Second Normal Form (2NF)To be in 2NF a table must:To be in 2NF a table must:– Be in 1NF (obviously)Be in 1NF (obviously)– Have all non key fields fully functionally dependant on the Have all non key fields fully functionally dependant on the

primary keyprimary key

In English:In English:– A non key field is one that is not part of the primary keyA non key field is one that is not part of the primary key– It means that you need to use the primary key to determine It means that you need to use the primary key to determine

the value of the other fields in the tablethe value of the other fields in the table– If you can find the value of other fields without using the If you can find the value of other fields without using the

primary key, you should remove that field from the table and primary key, you should remove that field from the table and place it in a separate tableplace it in a separate table

Third Normal Form (3NF)Third Normal Form (3NF)To be in 3NF a table must:To be in 3NF a table must:

–Fields which do not form part of the Fields which do not form part of the primary key must always be solely primary key must always be solely dependent on the primary key and dependent on the primary key and not on anything else, such as not on anything else, such as another non key fieldanother non key field

Third Normal Form (Cont.)Third Normal Form (Cont.)

Remove City from the table and create a new cities tableRemove City from the table and create a new cities table

Remove Salary from the table and create a job type tableRemove Salary from the table and create a job type table

Staff

EmployeeID*

LastName

FirstName

CityID

TypeID

Cities

CityID*

City

JobTypes

TypeID*

Salary

WardID #WardID #

No of BedsNo of Beds

Type of Type of WardWard

DoctorIDDoctorID**

PatientID#PatientID#

NameName

IllnessIllness

GenderGender

AddressAddress

DoctorIDDoctorID**

WardIDWardID**

DoctorID#DoctorID#

WardIDWardID**NameName

SpecialismSpecialism

ContactNoContactNo

DoctorTable (DoctorID#, Name, Specialism, Contact No, WardID,)

WardTable (WardID#, No of Beds,Type of Ward, DoctorID)

PatientTable (PatientID#, Name,Adress, Illness, Gender, DoctorID, WardID)

,

# Primary Key

* Foreign Key

Advantages of using databasesAdvantages of using databases(Advantages of normalisation)(Advantages of normalisation)

1.1. Avoids data duplicationAvoids data duplication-data stored once-data stored once-linked by keyfields-linked by keyfields-all data available via relational links in keyfields-all data available via relational links in keyfields

2. Controlled redundancy2. Controlled redundancy - Minimises data duplication - Minimises data duplication

3. Ensures consistency3. Ensures consistency of dataof data - of data to all users- of data to all users

4. Data independence4. Data independence - data stored separately from programs so can add new - data stored separately from programs so can add new fields because data is independent of the applications fields because data is independent of the applications which use itwhich use it

5. Increased security5. Increased security

What makes databases secureWhat makes databases secure ? ?

Hierarchy of passwordsHierarchy of passwords

User access rights and privileges User access rights and privileges

Data is stored separately from programs Data is stored separately from programs so different programs cannot overwrite so different programs cannot overwrite data.data.

DisadvantagesDisadvantages

Complex to set up and maintain; needs team of Complex to set up and maintain; needs team of programmers to maintain it.programmers to maintain it.

Database software is large complex expensive Database software is large complex expensive and requires powerful computers.and requires powerful computers.

All applications which access the data will be All applications which access the data will be affected if database fails. As the DBMS is the affected if database fails. As the DBMS is the only access to operational data a system failure only access to operational data a system failure can have serious consequencescan have serious consequences