1. Features of Relational Databases

2.  A structured collection of related data.
e.g.
 An address book
 A telephone directory
 A timetable

3. Databases are used to organise, store and retrieve
information as efficiently and effectively as possible.
Databases provide many benefits…
 Data is accessed more quickly
 Data has independence from the application
 Data can be shared between applications (so information only stored
once – applications ‗dip‘ into the data)
 Data redundancy is prevented (that is, duplication of data)
 Data more accurate due to validation
 Data can be queried – SQL
 Reports easy to generate - wizards
 Forms easily created (which help shield the user from the complexity of
the application)
 Storage of data more efficient
 Security of data better against intended/un-intended activity
 Backing up of data easy.
 Ensures all information is complete, accurate, relevant and timely.

4. Consider how information is stored in a
database File (Table)
– A set of related records
Record (Row)
– A collection of data about
an individual item
Field (Column)
– A single item of data
common to all records

5. Each column represents a field within the
record
Each line
represents
a record
within the
table

6.  Prior to relational database technology, all
databases were just files containing data.
 We call these flat files.
 Have a look at the example on the next
slide and see if you can see what the
problems are with having files like this…

8.  In the example for Sportswear Direct, five
suppliers have been used to provide
sportswear for the business
 Each time a purchase is made from a
supplier, their name and address details
have to be entered by the user
 Duplicating data entry can lead to
mistakes or errors, e.g. incorrect street
number or postcode

9.  A relational database could be used, which
contained a Supplier table
 Supplier details could then be entered just
once
 Each supplier could be given a unique
identifying code, e.g. a Supplier ID
 The supplier could then be referred to by this
unique code
 The code would become the ―Primary Key‖ of
the Supplier table

10. Supplier Table

11. Using Supplier ID rather than name and

12.  To further reduce the likelihood of incorrect
data and to reduce the amount of data
being keyed in, users could be given a
drop down menu to choose a supplier from
those available
 Users should not be able to key in a
supplier that does not exist on the Supplier
table

13. Carry out some research:
 Find a suitable definition of a relational
database
 Find more information about the purpose
of a relational database
 Include these, with references, in your
presentation

14.  We are now going to look at relational
databases in more detail. We will cover:
 Entities
 Attributes
 Relationships
 Benefits

15. ENTITIES:
 Entities are real world objects that are of
interest to the system, e.g. Customer
 You can hold information about these
objects
 Entities become tables in databases
 Can you think of any other entities?

16.  A Primary Key is a data item which is
unique and can be used to identify a
record with certainty
 AutoCounter data items are often used to
provide a primary key
 What are the problems with storing data in
a table like this?

17. In this example no field contains unique data as in each
field data is repeated at least once
It is not possible to identify a record by one field
So, we add an ID field to provide a unique identifier

18. Foreign Key
 When a primary key is copied to another
table to create a relationship, in the other
table it is called a Foreign Key
 Linked keys provide an Access Path to
establish if queries can be successfully
made
 Linked keys allow data to be located and
retrieved

19. PURCHASE SUPPLIER
TABLE TABLE
Purchase Order Supplier ID Primary Key
Foreign Key ID Name
Supplier ID Address
Date Postcode
Quantity Telephone
Item Number
Price
Using the example we used earlier, the Foreign
Key is the copy of the primary key from the
Supplier Table

20.  Referential Integrity is when the system is
checked to ensure that keys used in
relationships do in fact exist
 If a copy of a primary key is used as a link in
another table, then the value must exist in the
original table
 Using our example, the supplier code must
exist on the Supplier Table if it is used on the
Purchase Table
 This ensures consistency between the data in
linked tables
 Sometimes not all errors are found, and
manual checks of the data may be needed

21.  Auto Number data types can be used to
create a unique identifier for a table, e.g. for
a primary key
 The sequence of these numbers always
begins with the number one
 It is possible to set the initial value of a
number sequence by using an Append query
 This involves creating a temporary table
storing a single value showing where the
auto number should start

22. ATTRIBUTES:
 Attributes are the properties of an entity, that
is the information that you can hold about
them, e.g. name, address, telephone number
 Attributes become columns or fields in a
database table
 We will now look at some of the field
properties:
 Field names
 Data types
 Field properties
 Validation rules

23.  Microsoft Access gives you lots of flexibility when it
comes to naming your fields.
 A field can be 64 characters long
 A field can include any combination of letters,
numbers, spaces and special characters.
 There are exceptions – a period (.), an exclamation
point (!), an accent grave (`) and brackets ([ ]). You
cannot begin with a space.
 Ensure you give fields meaningful names – not
names internal to Access or VB as this may cause
conflicts
 Create table and field names without embedded
spaces – most SQL databases to which Access can

24.  Data types allow you to set properties and
thereby customise a field in a database
 In Access, data types appear with default
properties already in place
 Access supports 9 types of data – each
with a specific purpose
 There is however a 10th option, Lookup
Wizard, to help you define the
characteristics of foreign key fields that
link to other tables.

25. How the
table
looks in
Access,
showing
the data
types

26.  Text – normally selected for characters
 Memo – long strings of text (sentences &
paragraphs)
 Number – used for numbers but think
carefully about what you enter as the field
size property because this will affect
precision as well as length. E.g. Integers do
not have decimals
 Date/Time – useful for calendar or clock data
(allows calculations in minutes, seconds,
hours, days, months or years)
 Currency – always use for storing money
values. It has the precision of integers and

27.  AutoNumber – provides automatic
generation of primary key values (random
or sequential)
 Yes/No – used to hold true or false values
(useful for flagging accounts paid / not
paid etc)
 OLE Object – allows you to store
complex data such as pictures and graphs
etc.
 Hyperlink – allows you to store a simple
or complex link to an external file or

28. Property Description
Field Size Fixes the Max length of a text field
The default value is 50 characters
The maximum length is 255
Format This fixes how the data can be displayed
Input Mask This sets a pattern for the data to be entered
Caption This is the field label in a form or report
Default Value This is the value entered into a field when the record is created
It is usually left blank but can be very powerful
Validation Rule This defines the data entry rules
Validation Text This is the error message if data is invalid
Required This indicates whether an entry must be made or not.
If an entry is required, it is best not to set this property until the
database is fully working
Indexed This allows data to be sorted in the order of this filed which
speeds up searches
Allow Zero Length This is used with text fields to decide whether records in that
field are allowed to contain zero length or empty text strings

29. Source: Microsoft
http://office.microsoft.com/en-us/access-help/create-a-validation-rule-to-validate-data-in-a-field-
HA010096312.aspx [Accessed 28/04/11]
 A validation rule limits or controls what users
can enter in a table field or a control (such as
a text box) on a form.
 Microsoft Office Access 2007 provides a
number of ways to validate data, and you
often use several of those techniques to
define a validation rule. You can think of
validation rules as a set of layers — you can
use some or all of the layers when you need
to ensure that your users enter data properly.

30. Data types
 The data type restricts what users can enter.
For example, a Date/Time field accepts only
dates and times, a Currency field accepts
only monetary data, etc
Field sizes
 Setting a maximum field size can prevent
unnecessary data being entered, e.g. by
setting a field that stores first names to a
maximum of 20 characters can prevent an
inexperienced user from mistakenly entering
a first and last name in a field designed only
to hold a first name.

31. Table properties
 Table properties provide very specific types of
validation. For example, you can set the Required
property to Yes and, as a result, force users to
enter a value in a field.
Validation Rule
 Use this property to only allow specific values, and
the Validation Text property to alert your users to
any mistakes. For example, entering a rule such
as >100 And <1000 in the Validation Rule
property forces users to enter values between 100
and 1,000. The Validation Text property tells
users when they have made a mistake and how to

32. Input masks
 You can use an input mask to validate
data by forcing users to enter values in a
specific way. For example, an input mask
can force users to enter dates in a
European format, such as 2007.04.14.
Source: Microsoft
http://office.microsoft.com/en-us/access-help/create-a-validation-rule-to-validate-
data-in-a-field-HA010096312.aspx [Accessed 28/04/11]

33.  The term relationship is used to identify the
link between two database tables
 The link is created by creating a record that
contains the primary key from its own table and
a copy of the primary key from another table
 Identifying and creating these links is the most
fundamental requirements for database
designers
 Failing to implement the correct relationships
can have a detrimental effect on the efficiency
and functionality of the final database
 We will look at the example we used earlier

34. Purchase Order
Supplier ID
ID
Name
Supplier ID
Address
Date
Postcode
Quantity
Telephone
Item
Number
Price
Relationship
To link a supplier to a Purchase Order, the supplier‘s ID
is stored within the order. None of the other supplier
information is included on the purchase order.

35.  Relationships can be one of three types:
 One to one
 One to many, or many to one
 Many to many

36.  A success database requires all relationships
to be defined as one to many (or many to
one) relationships.
 Any many to many relationships must be
resolved until they become one to many (or
many to one)
 One to one relationships often mean that one
set of data belongs to the other in the link
and is therefore an attribute

37.  Normalisation helps to correctly design
relational databases.
 It is designed to:
 Remove repeating data (so data is stored more
efficiently)
 Remove redundant data
 Remove many to many relationships
 Help identify the keys (primary key and foreign
key)
 Normalisation will be covered in more detail

38.  Reduced data redundancy
 Reduced data storage
 Faster access
 Ease of data extraction
 Efficient updating
 Efficient searching
 Efficient sorting
 Efficient reporting