Computing The Teacher

Database Design

CP4 Revision

Computing The Teacher

Database Design

• Careful database design is needed to avoid…

• Data Redundancy – storing the same data item in more than one place (waste of storage space)

• Data Inconsistency – two versions of the same data may be different.

Computing The Teacher

Entities

• An entity is a thing about which data is stored (Eg Customer, Employee, Stock)

• A relationship is a link between two entities.

Computing The Teacher

Relationships

• One-to-One : One Pupil has one Network Account.

• One-to-Many : One Pupil borrows many LibraryBooks

• Many-to-Many : One Teacher teaches many Pupils – One Pupil is taught by many Teachers.

Computing The Teacher

Entity-Relationship Diagrams

Pupil AccountOne-to-One

One-to-Many

Many-to-Many

Teacher Pupil

PUPILS LibraryBooks

Computing The Teacher

Key Fields

• Each Entity must have a unique key field – the primary key.

An Entity may have foreign keys – primary key fields of other entities. These provide the links (relationships) between different entities.

WardID is a foreign key.

PatientID Surname Forename Disease WardID

P101 Smith Paul Measles W10

P102 Jones Jane Mumps W12

P103 Wilkins Mary Measles W10

P104 Jackson Tom Tonsillitis W17

PatientID is the primary key field.

Computing The Teacher

Table Design

• The standard notation for writing down the design of a table…

• CAPITAL letters for the name of the table

• Underline the key field,

• Italicise the foreign keys.

• Eg.:PATIENTS (PatientID, Surname, Forename, Disease, WardID)

Computing The Teacher

Database Normalisation

• Avoids data duplication

• Avoids data inconsistencies

• …ensures the best possible design for a database so other applications can use it.

Computing The Teacher

Un-Normalised Data

PupilID PupilName DOB ExamID Subject Level Date RoomID RoomName

P99010 Jane Grey 12.03.86 CP101

EN004

AR075

Computing

English

Art

AS

GCSE

AS

15.05.01

24.05.01

12.06.01

UH

UG

UG

Hall

Gym

Gym

P99205 Tom Jones 05.11.86 CP101

MA110

PH190

Computing

Maths

Physics

AS

AS

AS

15.05.01

15.06.01

08.06.01

UH

UG

58

Hall

Gym

Science Lab

P99311 Sam Hill 16.08.86 CP101

EN004

Computing

English

AS

GCSE

15.05.01

24.05.01

UH

UG

Hall

Gym

There are a number of repeated fields here…

To change into First Normal Form – the repeated groups of fields must go…

Split into two linked tables…

Computing The Teacher

First Normal Form• PUPILS ( PupilID, PupilName, DOB,

ExamID)

• EXAMS ( ExamID, Subject, Level, Date, RoomID, RoomName)

• The key field of the PUPILS table is a combined key field.

• The link is made through the foreign key ‘ExamID’.

PupilID PupilName DOB ExamID

P99010 Jane Grey 12.03.86 CP101

P99010 Jane Grey 12.03.86 EN004

P99010 Jane Grey 12.03.86 AR075

P99205 Tom Jones 05.11.86 CP101

P99205 Tom Jones 05.11.86 MA110

P99205 Tom Jones 05.11.86 PH190

P99311 Sam Hill 16.08.86 CP101

P99311 Sam Hill 16.08.86 EN004

ExamID Subject Level Date RoomID RoomName

CP101 Computing AS 15.05.01 UH Hall

AR075 Art AS 12.06.01 UG Gym

MA110 Maths AS 15.06.01 UG Gym

PH190 Physics AS 08.06.01 58 Science Lab

EN004 English GCSE 24.05.01 UG Gym

To get this into Second Normal Form, no field must be dependant on only part of the key field.

Do this by creating a linking table

HINT : You need to do this every time you have a Many-to-Many relationship)

Computing The Teacher

Second Normal Form• PUPILS ( PupilID, PupilName, DOB)• EXAMS ( ExamID, Subject, Level, Date, RoomID,

RoomName)• PUPIL_SITS(PupilID,ExamID)

PUPILS EXAMS

PUPILS EXAMSPUPIL_SITS

…must be changed to…

The Many-to-Many relationship…

Computing The Teacher

Second Normal Form

PupilID PupilName DOB

P99010 Jane Grey 12.03.86

P99205 Tom Jones 05.11.86

P99311 Sam Hill 16.08.86

ExamID Subject Level Date RoomID RoomName

CP101 Computing

AS 15.05.01

UH Hall

AR075 Art AS 12.06.01

UG Gym

MA110 Maths AS 15.06.01

UG Gym

PH190 Physics AS 08.06.01

58 Science Lab

EN004 English GCSE 24.05.01

UG Gym

PupilID ExamID

P99010 CP101

P99010 EN004

P99010 AR075

P99205 CP101

P99205 MA110

P99205 PH190

P99311 CP101

P99311 EN004

To make this database into Third Normal Form, there must be no non-key dependencies…

..so separate the Rooms into another table…

Computing The Teacher

Third Normal Form

PupilID PupilName DOB

P99010 Jane Grey 12.03.86

P99205 Tom Jones 05.11.86

P99311 Sam Hill 16.08.86

PupilID ExamID

P99010

CP101

P99010

EN004

P99010

AR075

P99205

CP101

P99205

MA110

P99205

PH190

P99311

CP101

P99311

EN004

ExamID Subject Level Date RoomID

CP101 Computing

AS 15.05.01

UH

AR075 Art AS 12.06.01

UG

MA110 Maths AS 15.06.01

UG

PH190 Physics AS 08.06.01

58

EN004 English GCSE 24.05.01

UG

RoomID RoomName

UH Hall

UG Gym

UG Gym

58 Science Lab

UG Gym

PUPILS (PupilID, PupilName, DOB)

EXAMS (ExamID, Subject, Level, Date, RoomID)

PUPIL_SITS (PupilID, ExamID)

ROOMS (RoomID, RoomName)