18 database features

Unit 18 Database Design

Features of Relational Databases

A structured collection of related data. e.g.

An address book A telephone directory A timetable

What is a Database?

What is a Database?

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.

What is 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

Consider how information is stored in a database

What is a Database?

Each column represents a field within the record

Each line represents a record within the table

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…

What is a Relational Database?

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


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



Supplier Table


Using Supplier ID rather than name and address

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


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


We are now going to look at relational databases in more detail. We will cover:

Entities

Attributes

Relationships

Benefits

Features of Relational Databases

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?

Entities

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?

Entities – Primary Keys

Entities – Primary Keys

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

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

Entities – Foreign Keys

Entities - Foreign Keys

PURCHASE TABLE

Purchase Order ID

Supplier IDDate

QuantityItemPrice

SUPPLIER TABLE

Supplier IDName

AddressPostcodeTelephone Number

Using the example we used earlier, the Foreign Key is the copy of the primary key from the Supplier Table

Foreign Key

Primary Key

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

Entities – Referential Integrity

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

Entities -Auto-incremented Keys

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

Attributes

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 attach do not support spaces within names.

Attributes – Field Names

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.

Attributes – Data Types


How the table looks in Access, showing the data types

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 allows decimal places to be changed


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 document e.g. this could be a URL.


Attributes – Field Properties

Property DescriptionField Size Fixes the Max length of a text field

The default value is 50 charactersThe maximum length is 255

Format This fixes how the data can be displayedInput Mask This sets a pattern for the data to be enteredCaption This is the field label in a form or reportDefault 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 rulesValidation Text This is the error message if data is invalidRequired 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

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.

Attributes – Validation Rules

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.


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 fix the error.


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]


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

Relationships

Relationships

Purchase Order ID

Supplier IDDate

QuantityItemPrice

Supplier IDName

AddressPostcodeTelephone Number

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.

Relationships can be one of three types:

One to one

One to many, or many to one

Many to many

Relationships

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

Relationships

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 in Assignment 2

Relationships

Benefits of Relational Databases Reduced data redundancy Reduced data storage Faster access Ease of data extraction Efficient updating Efficient searching Efficient sorting Efficient reporting

18 database features

Documents

data data redundancy

duplication of data

unique data

validation data

application data

data easy

benefits data

eachfield data