Download Chapter 43 – Relational databases

AQA A level Computer Science
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Chapter 43 – Relational databases
TASK QUESTIONS (WITH ANSWERS)
1
What is a relational database?
A relational database is a way of storing data in any number of related tables. Separate
tables are created for each aspect of the database and then links are created between the
tables.
2
Explain the following terms:
a)
entity
a table of data stored in a relational database
b)
attribute
a field in a relational database
c)
relationship
a link between one table and another in a relational database
3
Describe how primary keys and foreign keys are used to create relationships
between tables.
A primary key is an attribute that uniquely identifies every record in an entity.
To create a relationship between entities, the primary key from the first entity is included in
the second entity as the foreign key.
Therefore the relationship is created by joining the primary key in the first entity to the
foreign key in the second.
4
A company employs engineers to fix faults on photocopiers. The company has 20 engineers
and over 200 clients. The engineers will travel to client sites in order to fix their machines
and can fix between one and five machines a day. The company employs a ‘work controller’
who takes the call from the client and then allocates an engineer to the job. She uses a
relational database to keep track of all the details relating to the clients, engineers and jobs.
Part of the database is shown below in standard notation:
CLIENT (Name, Address)
ENGINEER (Name, Address)
JOB (Date, Nature of Problem)
a)
Suggest a suitable identifier for each table.
CLIENT, ENGINEER and JOB
AQA A level Computer Science
© Hodder & Stoughton Limited 2015
AQA A level Computer Science
Teaching and Learning Resources
b)
c)
d)
e)
f)
5
Draw an entity-relationship diagram to show the relationship between the
three entities.
CLIENT
JOB
ENGINEER
One client has many jobs
One engineer has many jobs
Suggest foreign keys that could be used to create the relationships.
Client ID and Engineer ID
Identify four other attributes that the company may store.
Client’s details such as company name, address, contact details, make of copier.
Engineer details such as name and address.
Job details, such as the nature of the fault, date recorded, date fixed.
Now complete the standard notation.
CLIENT (Client ID, Company Name, Contact Name, Address, Type of copier)
ENGINEER (Engineer ID, Name, Address)
JOB (Job ID, Client ID, Engineer ID, Date recorded, Date fixed, repair carried out)
How could attributes be combined to form a composite key for the ENGINEER table?
A candidate key must uniquely identify each engineer. If an Engineer ID is not used
then name alone would not be sufficient as you could have two engineers with the
same name. Two attributes could be combined, for example, Engineer Name and the
first line of the address to ensure it is unique.
A garage uses a database to store details about its customers, their cars and the repairs that
are carried out. The system is currently stored as a single table, an extract of which is shown
below.
a)
b)
Identify three problems with the way the database is currently stored.
Data is repeated, the data is not normalised, the database is going to get large and
disorganised
A relational database is to be designed using three tables: Customer, Car and Repairs.
Normalise the database to 3NF showing your answer in standard notation:
ENTITYNAME (Primary key, attribute 1, attribute 2 …)
This will involve identifying suitable primary and foreign keys.
AQA A level Computer Science
© Hodder & Stoughton Limited 2015
AQA A level Computer Science
Teaching and Learning Resources
CUSTOMER (Customer ID, Name, Address, Job Number)
VEHICLE (Registration Number, Make, Model, Year)
JOB (Job Number, Customer ID, Registration Number, Nature of repair, Date, Price)
Customer
Address
Reg No
Make of car
Date repaired
Repair carried out
John Brown
Mary Jones
John Brown
Jane Fox
1 High Street
10 Low Road
1 High Street
2 New Lane
M222 HGG
K222 HKK
P333 AAA
J123 AAA
Ford Escort
VW Golf
Citroen Saxo
VW Polo
11/03/15
11/03/15
11/03/15
11/03/15
Replace exhaust
Electrical fault
New tyres
Starter motor
AQA A level Computer Science
© Hodder & Stoughton Limited 2015