Databases Notes

8/9/2019 Databases Notes

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Computer Studies Notes

DATABASES

i. What is a database?H

A database is a collection of data related to a particular subject or purpose.Amongst others, databases are used in:

Tracking customer orders

Maintaining a music collection

Keeping class and student records in a school

Keeping a list of online sites as done by search engines

The software which manages the database file, such as MS Access is called

a Database Management System (DBMS). It is important to note that adatabase is a file, as a document or a spreadsheet whilst a DBMS is a

software like Microsoft Word, Microsoft Excel and so on.

The DBMS software is designed so that people can keep records relatively

easily on computer. Users can add, modify or retrieve data using search

conditions and create relevant reports with ease.

ii. Opening a Database using MS AccessH

1. Click the Start Button

2. Choose Programs

3. Click Microsoft Access

4. Select Blank Database

5. Select the drive and folder where the

database file will be saved.

6. Enter a name for your database file

7. Click the Create button

The 6 objects (Tables, Queries, Forms, Reports, Macros and Modules)

making up a database are shown on screen. Clicking on the respective object

one can see different items. Before you start entering your data in a

database you have to sit down and write down what details you want to put in

Ms M. Cristina Page 1


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your database i.e. you have to establish the fields needed. Following this

task you can create a table to contain your data.

There are mainly two database views:

1. Design View- Used by the person that creates a database

- Used to design the table / form / report

- Set up the fields and their properties for the Table of data

2. Datasheet View

- Used by the person entering data into a

database

iii. Creating a new Table

A table holds data about a particular item. The following print screen is an

example of a students table:

To create a new table:

1. Click the Table tab from the database window

2. Click Create Table in Design View



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iv. Creating Fields

Prior to explain what is a field, consider the following form:

This form shows, and lets you manipulate data about a single recording, in

our case a single student. Such data is called a record. A record is the

basic unit of data stored in a data file. It is a collection of item, all relating

to the individual or object. Records are treated as a unit for processing.

A record is made up of fields each representing an attribute of the record.

Examples of such fields include the Name and Surname fields. A field is

part of a record designed to hold a single data item. Each field is referred

to by a field name, which identifies the data in the field and makes it

possible to generalize about the data being processed.

The following is an example of a typical field:

As you can notice forms represent data in a convenient form for browsing

and updating. Each form is a view of some data, together with facilities to

navigate the data, add, edit and delete records, etc.


Field

Name

Field

Contents


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Navigating the database using forms:

You can move from record to record using navigation buttons provided at the

bottom of the form:

Move to the previous record in the

record set

Move to the next record in the

record set

Move to the first record in the

record set

Move to the last record in the

record set

Note that Student ID field changes as you move from record to record.

The Student ID is a number identifying a record, since no two students will

have the same student ID. It is the key field of our student collection.

What is a key field?

Must be a unique field

Distinguishes each record from another

It uniquely identifies each record

Updating a Record

You can update the contents of a field within a record by clicking on the

field contents and typing in the new data.

Adding / Inserting a new record

As you move forward past the last record in the record set, you will arrive

at a blank record ready for you to fill in.


To add a new record in the database


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Note: the Student ID field (which is the key field of the record) appears as

Autonumber on the new record. This is because this field is not directly

editable. Instead the designer of the database has chosen to let MS Access

assign a unique ID number to each record.

v. Designing our database

1. Click the Table tab from the database window

2. Click Create Table in Design View

3. On each line of the top half of the table window, enter the name of a

field, its data type and a longer description to help you identify the

field

4. The properties for each field appear in the bottom half of the table

window. You can move between the top and bottom panes by pressing

F6.

5. Create a key field by clicking on the selector, the small box at the

beginning of the row for that field.

6. Click the Set Primary key button.

The key symbol is displayed to represent the key field from the whole list:



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7. When you finish, click on the save button. Enter a name for the table

and select OK. If you did not select a key field, Access prompt you

about creating a Primary Key. Click the Yes button to have Access

create an AutoNumber field and make it primary key, or No to avoid

creating a primary key.

vi. Data Types

Access has 10 types of fields to store specific types of data. These include

the following:

Data Type Description

Text Allows you to enter information in a field that will not becalculated such as the telephone number.

Text fields can contain up to 255 characters.

Memo Allows you to enter up to 64,000 characters

Number Allows you to enter numbers in a field that you can calculate

such as marks

Date/Time Allows you to enter date and time values from the year 100 to

the year 9999

Currency Allows you to enter numbers having a currency symbol.

Access displays two decimal places, but you can change thedefault.

AutoNumber Allows you to display a field that counts the number of

records in a table. Whenever you enter a new record, the

counter for that record registers one higher than the

previous record number. Only one counter field is allowed in a

table.

Yes/No Allows you to enter either a Yes or No

OLE Object Allows you to link or embed objects such as graphs and

picturesHyperlink Text or combinations of text and numbers stored as text as a

hyperlink address.

Lookup

Wizard

Creates a field that allows you to choose a value from another

table or from a list of values.



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You can set the Field Size property to specify how many characters a text

field can contain or the range of numbers that a Number field can contain.

For example, if you know that a Text field is going to contain first names,

you may want to set its Field Size property to 15. You are unlikely to

encounter a first name of more than 15 characters.

vii. Database Operations

Inserting Fields

1. Highlight the row where the new row will be inserted2. Click the Insert Row button

Deleting Fields

1. Highlight the row to delete

2. Click the Delete Row button

Moving Fields

1. Highlight the row/s to move

2. Position the mouse pointer on the selector box of the row/s to move

3. Drag the bold line, which appears to the appropriate position where

the row/s are to be inserted


Field size was

changed from

50 characters

to hold 15.


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viii. Entering Data in a Database

So far you have been working in the Tables Design View. You cannot enter

the actual data in this view.

1. Click the save button

2. Type a name for the table

3. Click the Ok button

4. Click the Datasheet view button

5. Type the data in the appropriate cells

6. Press the TAB key to move from one cell to the next.

Editing Records

1. Click in the cell where you want to edit the data

2. Edit as appropriate

When you are editing a record, Access displays a pencil icon in the left-mostcolumn of the datasheet. When you move to another record making changes

to the existing record, Access saves the edited record.

Deleting Records

1. Highlight the row to delete

2. Press Delete key

3.

Click theYes button



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ix. Field Properties

Apart from specifying types in the Table Design View you can also set field

sizes, input masks, formats and indexes. Formats and input masks affect

the way data appears in a table in Datasheet. However, they serve verydifferent purposes.

Field Sizes

Ideally you should set each field size to contain a specific amount of

characters or numbers. You should generally use the smallest possible Field

Size setting because smaller data sizes can be processed faster and require

less memory. The field size sets the maximum size of data that can bestored in a field.

Note that:

If you change the Field Size settings for a text

field from 255 to 50, data over 50 characters will be lost

If the data in a number field doesnt fit in a

new setting range, fractional numbers may be rounded or you

might get a Null value.

Format Property

The format property setting affects the data in the field once it is entered.

It changes how the data displays and can make it easier to read. For

example:

If you apply the Long Date format to a

Date/Time field, a date of 11,24,94 displays as Thursday,November 24, 1994.

If you want to display the data in uppercase you

type > in the Format box



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x. Data Input Validation

There is a famous principle in computing that garbage in garbage out-

sometimes referred to as the GIGO principle. Errors in data entered into acomputer system will propagate throughout the system and result in

incorrect output. Thus, correctness of data is one of the major concerns in

any database application, and it is vital that checks are performed on the

data entered into the database to minimize data entry errors.

Here we are principally interested in validation checks, which the DBMS can

automatically perform on data entered into a field. The following are some

such common checks:

Type Checking

The DBMS can check that data entered into a field is of the

correct type. For example, only numbers are allowed in a numeric

field, and dates in a date field are checked to ensure that they

are valid (example: 30th February is not allowed)

Note: there is a distinction between Validity and Correctness.

A date of birth for example may be valid but still incorrect.

Type checking is the most basic form of data input validation, andis performed automatically by the DBMS. All other checks must

be specified by the database designer using a database

validation rule.

Range Checking

The DBMS can check that the value entered in a field is within a

certain range otherwise it will be rejected. The range must of

course be specified by the designer of the database, since the

DBMS has no knowledge of what range is considered acceptable

in a particular application. Thus, if we have an age field in our

database, this field may be specified to accept values between 11

and 16, sp the DBMS will reject numbers outside this range.



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Set Checking

The DBMS can check that the user only enters value from a

predetermined list of acceptable values, for example in a bankingdatabase application: the account type may have to be one of

Current, Savings or Fixed. Having the user select the value from

a drop-down list can enforce this.

xi. Searching and Sorting records

Before searching and sorting fields, it is important to set indexes to fields.

Indexes are commonly used in books to look for any particular informationquickly. Similarly, setting an index to a field speeds up searching and sorting

on the fields, but this may slow updates.

Indexing a Field

1. Click the Design View button

2. Click the appropriate field to be indexed

3. Click in the Indexed Property field box to see a drop down menuwith a list of options

4. Click the appropriate option

5. Click the save button


Option: To do this:

No Deletes the existing index or leaves the field unindexed.

Yes (Duplicates ok)Indexed the field, allowing duplicates values among

fields n the table.

Yes (No Duplicates) Indexes the fields, prohibiting duplicate values amongfields in the table.


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Searching Data

Information Retrieval also known as data retrieval is extracting useful

information from large amounts of stored data, such as a database. Thisenables large amounts of data to be searched and the results to be

outputted efficiently.

One purpose of a database is to provide quick and efficient access to data.

1. Click the Datasheet View button

2. Click the appropriate field to find the data

3. Click the Find button

4. Type the string to be found in the following dialogue box

5. In the Find What section type the data you are looking for.

6. In the look In section ensure that the correct field is

displayed

7. Click on the Find next button

Sorting Data

Sorting is the process of arranging a set of records in order in some field of

interest. Thus, you may wish to arrange a set of records in ascending order

by surname, or in descending order by marks obtained in a test.

Files are sorted because:

Files are easier to update and merge if they are sorted in the

same order

Searching a sorted file may be easier



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People find sorted files easier to read and use

How to sort data:

1.Click the Datasheet View button

2. Click anywhere in the field to sort

3. Click the sort ascending or sort descending button

xii. File Structure

The way in which a file is to be accessed by users influences how it is

stored, and the information the systems software needs to hold in order to

allow for efficient reading and writing.

Serial Access

Items are read, one at a time

from the physical start of file, in the order

which they are stored.

Items may vary in length, provided there is

some marker to signal the end of one item and the start of the

next. Usually data records are placed on a file in a

particular order (e.g. alphabetical) according to a sort key. This

is known as a sequential file.

Direct Access

Items can be retrieved immediately provided

its position in the file is known.

Items must have a known length,

so that software can calculate where in the file

the required item is located.



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Direct access files are commonly called random

access files because they are usually held in storage that has

random access.

Advantages of sequential access over direct access

Low-cost media can be used.

Automatic back-up files (old master files on

separate tape).

It is usually easier to write programs, which

handle sequential files.

Advantages of direct access over sequential Amendments and updates can be made to

individual records quickly and on demand.

Selected records can be accessed far more

quickly in any order.

Records do not have to be stored in any

particular order.

Several files can be processed or updated at

the same time.

xiii. Queries

A query is a question asked of the data in a database. It retrieves data

from one or more tables using criteria you specify, and then displays it in

the order you want.

The query is structured so that the answer is either true or false. To

answer the query the computer must check each record of the data to see if

the answer is true. The result of the query is a list of all the data, which

satisfies the query.



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For example, the following query result shows only those records of

students who live at Birkirkara:

Creating a query

A query cannot be created unless a database file with an existing table has

been created.

The Design View will be used to create a query. Design View shows thetable and fields on which the query is based, and the criteria to be used to

select the required data from those tables, as well as any sorting to be

performed on the data. The query design screen is really a simple tool for

creating queries the queries itself is actually a small program written in a

specialized language called SQL (Structured Query Language).

1. Click on Queries in the Objects

2. Select Create Query in Design View

3. Select the table on which you want to do your query (e.g.

Students Table)

4. Select the table and click Add

Access creates a new query object and the query design window. This

window is divided into 2 areas:



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Table Pane

Includes boxes for the table/s you have added to your query. These boxes

contain a field list, which is a list of the available fields in each table.

QBE (Query by example) Grid

This is a collection of cells where you define the query. The 1st row is used

for the querys field names, the 3rd row (Sort) for sorting options and the 4th

row (Show) to determine which fields appear in the query results. The rest

of the rows (Criteria) are used to set up your criteria.

5. From the field list, double click on the fields you want to use. The

order in which you select your fields is the order in which they

appear in the dynaset (a datasheet containing the results of aquery).

6. Click the drop-down arrow in the Sort box and select the sorting

order for the data in that field.

7. In the Criteria box, type the required expression.

8. Deactivate any check boxes in the Show cells. In most cases

fields are added to the QBE grid for criteria purposes only. Such

fields are not necessarily important to be displayed in the dynaset.

9. Click the Run button to display the results of the query.

10. Save your query.

Criteria Expressions


1. Table

Pane

2. QBE

grid

Field

List


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Criteria expressions are short statements that tell Access what subset of

your data you want to see.

Symbol Description

= Equal to< Less than

Greater than

>= Greater than or equal to

Not equal to

Examples:

Expression Field Description

Borg Surname Displays records where Surname is Borg

John Name Displays records where Name is not equal to John

= 45 Maltese Displays records where the Maltese mark is greater or equal

to 45

> #1/1/95# Date of Birth Displays records where the date of birth is after the 1st of

January 95

Null Religion Displays records where the Religion field is empty

Example of a query using a single criterion:

Running the above query displays all students who live at Birkirkara.

Example of a query using multiple criteria



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Running the above query displays all students who live at Birkirkara and have

a Student ID which is greater than 5.

xiii. Forms

Forms let you manipulate data about a single recording.

1. From the objects menu, select Forms

2. Double click on Create Forms using Wizard

3. Select the fields you would like to include in the form and click

on the Next button

4. Select the layout from your form and click on the Next button

5. Select the style for your form and click on the Next button

6. Give a title to your form and click on Finish button

The following is an example of a generated report:



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xiii. Reports

Once you have created your table and entered the records, you might want

to extract and print specific information from your database. For example

you might want to print students progress reports or generate mailing labels

for students living in a particular village.

The process of selecting and printing information is done through reports.

Report is one of the Microsoft Access database objects that presents

information formatted and organized according to your specifications.

Creating a Report with a Wizard

With a wizard, you can quickly create a basic report that shows all the fields

and records in a table or query, or you can create a custom report that

displays data from selected fields in a particular format such as mailing

labels.

1. Click on Reports from the Object list.

2. Click Create Report by using Wizard.

3. In the Table/Query box, type the name of the reports source

table or query, or select one from the list.

4. Select the fields to include on your report. Click on the Next

button.

5. Set any grouping levels as necessary.

A grouping level is a field upon which the records are grouped. Access

determines the unique values in the field, and any records that have the

same value in that field are displayed together in the report.

In the following example, the generated report is grouped by the class field.



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6. Determine how the fields selected will be sorted. Click the

next button.

7. Enter the appropriate options (Columnar, Tabular or Justified,

Portrait or Landscape) to set the layout of the report. Click

the next button.

8. Select a style for your report e.g. Bold, Casual and Compact etc.

Click the next button.

9. Type a title for your report. Click the finish button.



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