DESIGNING A DATABASE

MS ACCESS

www.icplus.org (McRoyal)

The Design Process

• The design process consists of the following steps:

▫ Determine the purpose of your database

This helps prepare you for the remaining steps.

▫ Find and organize the information required

Gather all of the types of information you might want to

record in the database.

▫ Divide the information into tables

Divide your information items into major entities or

subjects. Each subject then becomes a table.

The Design Process (cont’d)

• Turn information items into columns

▫ Decide what information you want to store in each table.

Each item becomes a field, and is displayed as a column in

the table.

• Specify primary keys

▫ Choose each table’s primary key.

• Set up the table relationships

▫ Look at each table and decide how the data in one table is

related to the data in other tables. Add fields to tables or

create new tables to clarify the relationships, as necessary.

The Design Process (cont’d)

• Refine your design

▫ Analyze your design for errors. Create the tables and

add a few records of sample data. See if you can get the

results you want from your tables. Make adjustments to

the design, as needed.

• Apply the normalization rules

▫ Apply the data normalization rules to see if your tables

are structured correctly. Make adjustments to the tables,

as needed.

Normalisation

• Normalisation is the process of breaking up larger

relations into many small ones using a set of rules.

• The process involves identifying functional

dependencies. If they are found to be attributes that

are not directly dependent on the PK, these are

extracted to form new relations.

Normalisation (cont’d)

• The process is carried out until all the data in each

relation is clearly and uniquely associated with other

data in the same table. This reduces redundancy

(although it does not eliminate it) and makes the data

easier to maintain.

Normal Forms

• A number of normal forms have been proposed but

the first five normal forms have been widely accepted.

• The norm forms progress from 1NF, to 2NF, and so

on. Data in 2NF implies that it is also in 1NF – i.e.,

each level of normalisation implies that the previous

level has been met.

• Other normal forms such as Boyce-Codd (BCNF)

which is an extension of 3NF, 4NF and 5NF also exist

First Normal Form (1NF)

• A relation (table) that contains a repeating group (or

multiple entries for a single record) is called an

unnormalised relation.

• Removing repeating groups is the starting point in the

quest to create tables that are as free of problems as

possible. Tables without repeating groups are said to be in

1NF

• A relation is in 1NF if – and – only if – all domains contain

only atomic or single values, i.e., all repeating groups of

data are removed.

1NF(cont’d)

• In order to convert an unnormalised relation into

1NF, first identify the key attribute(s) involved. Also,

identify a key for the whole relation.

• Make a separate relation for each group of related

attributes

• Give each new relation a primary key

Second Normal Form (2NF)

• A relation is in 2NF if

▫ it is in 1NF

▫ and all non-key attributes are dependent on the whole of

the primary key and not part of it.

• If an attribute depends on only part of a multi-value

key, remove it to a separate table.

Third Normal Form (3NF) • A relation is in 3NF if

▫ it is in 2NF

▫ non-key attributes are dependent on the primary key and

independent of each other. i.e., non-key attribute must be non-

transitively dependent on the primary key.

▫ a non-key attribute is changed, that change should not affect others

• Make a separate relation for attributes transitively dependent

on the primary key.

• Give each new relation a primary key

• Original relation will include a foreign key to link to new

relation

Example

• Help At Home is an agency which provides various

services such as baby sitting and dog walking. Details

of all bookings and a record of the service carried out

are kept. Assume only one person(employee) is

involved in carrying out each particular service. An

hourly rate is charged depending on the type of service

required. Note that customers will only be able to

make one booking for a particular service per day.

Example (cont’d)

Customer name:

Tel: 081 980 2223

John Smith Customer no: 111

Date of booking: 18.09.06

Type of service required: 02 Service required: Dogwalking

Date service is required: 30.09.06

Person carrying out job: Ben Brown

Charge per hour: $10 Hours: 1

Customer name:

Tel: 081 980 2223

John Smith Customer no: 111

Date of booking: 11.08.06

Type of service required: 01 Service required: Baby sitting

Date service is required: 16.08.06

Person carrying out job: Issa Ahmed

Charge per hour: $7 Hours: 5

Book record example 1

Book record example 2

Example (cont’d)

Relation in un-normalised form

Cus_No Cus_name Cus_Tel Booking_date Service_Type Service Date_required E_no E_name Per_hr_

$

Hrs

111 J Smith 980

2223

11.08 01 Babysit 16.08 1 Issa

Ahmed

7 5

18.09 02 Dogwalking 30.09 2 Ben

Brown

10 1

112 G Best 678

4455

12.06 01 Babysit 13.06 2 Ben

Brown

7 3

Example (cont’d) – 1NF

• Identify PK – Cus_no, Booking_date, Service_type

• 1NF determinacy diagram

Cus_no

Booking_date

Service_type

Cus_Tel

Cus_name

Date_required

E_name

Hrs

Service_required

Per_hr_$

E_no

Example (cont’d) – 1NF

• Relation in 1NF

Cus_No Cus_name Cus_Tel Booking_date Service_Type Service Date_required E_no E_name Per_hr_$ Hrs

111 J Smith 980

2223

11.08 01 Babysit 16.08 1 Issa

Ahmed

7 5

111 J Smith 980

2223

18.09 02 Dogwalking 30.09 2 Ben

Brown

10 1

Example (cont’d)- 2NF

• 2NF determinacy diagram

Example (cont’d) – 2NF

• The relation in 2NF

Cus_No Cus_name Cus_Tel

111 J Smith 980 2223

112 G Best 678 4455

Service_Type Service Per_hr_$

01 Babysit 7

02 Dogwalking 10

Cus_No Booking_date Service_Type Date_required E_no E_name Hrs

111 11.08 01 16.08 1 Issa

Ahmed

5

111 18.09 02 30.09 2 Ben

Brown

1

Example (cont’d) – 3NF

• 3NF determinacy diagram (the service type and

customer diagrams remain the same as in 2NF)

Example (cont’d) – 3NF • The relation in 3NF (the service type and customer tables

remain the same as in 2NF) Cus_No Booking_date Service_Type Date_required E_no Hrs

111 11.08 01 16.08 1 5

111 18.09 02 30.09 2 1

Cus_No Cus_name Cus_Tel

111 J Smith 980 2223

112 G Best 678 4455

Service_Type Service Per_hr_$

01 Babysit 7

02 Dogwalking 10

E_no E_name

1 Issa Ahmed

2 Ben Brown

Developing a database in MS Access

• Practical session to be taken in class

Query-By-Example (QBE)

• QBE is an approach to writing queries that are very visual.

• With QBE, users ask their questions by entering column names and other criteria using an on-screen grid, and data appears on the screen in tabular form

• In Access 2007 & 2010, queries are created using the Query Wizard or Query Design in the queries groupings under the Create tab

• Practical session to be taken in class

Examples

1. List the number, name, balance and credit limit of all

customers in the database

2. List all fields and all rows in the Orders table

3. Find the name of customer 148

4. List the description, on hand value and warehouse

number for all parts that have more than 10 units on

hand and that are located in warehouse 3

5. List the description, on hand value and warehouse

number for all parts that have more than 10 units on

hand or that are located in warehouse 3

6. List the number, name and balance for each customer

whose balance is between £1000 and £5000