databases - bh

8/13/2019 Databases - BH

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Databases

GCSE


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Using a Database

Search and retrieveinformation or data that

has been collected andspecially organized.


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A Database Management System

(DBMS)

System of planning, creating and maintaining a database1. Tables and relationships2. Field types

Text Numeric

Real Decimal

Date Boolean Lookup

3. Field properties file size as small as possible

no wasted storage space. reduces time to process data.

fixed and variable field lengths4. key field or primary keyunique5. Set up queries and Reports6. Determine Access rights


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Common field types found in databasesUse this field type When the field's data is

Text Text - up to 255 characters

Memo up to 64000 characters

Number (numeric field) Numbers, for example 12345

Date Dates, for example 21/04/55

Currency or $Autonumber Automatically generates a number every

time a new record is created

Object container Picture, video clip, sound file

Yes/No (also known as aBoolean or logical field)

Yes or No

Calculation Result of a formula,


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Card index becomes a database


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These are the fields

there are fourfields in each record of this file.

This is one complete record- there are fiverecords in this file.

ISBN is the key field. This is the field that has a different value in everyrecord. It is used to distinguish one record from another. Some bookscould have the same title, author or publisher. The ISBN is the only way aparticular book can be picked out.

A key field uniquely identifies an individual record.

Structuring Data


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Fixed LengthRecords

A fixed length record - the length of the fields in each record has been

set to a maximum number of characters long.

A set amount of storage space is set aside for each field.

M r

B o l d

D a m o n

3 1 P a r k L a n e


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Advantage start and end of each record is a fixed number of characters apart

- much easier to locate records and fields.

Disadvantages:- Fields rarely contain maximum number of characters allowed -

wastes space. Values sometimes cant be entered - too large to fit inside the

allowed space in a field.

Fixed Length Records


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Avariable length recordthe length of a field can change

to allow data of any size to fit.

Aspecial marker (#in this example) indicates where eachfield ends.

M r # D a m o n # B o l d #

3 1 P a r k L a n e #

Variable Length Records


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Advantage: space is not wasted, only the space needed is used.

Disadvantage more difficult to locate the start and end of records

and fields.

When records need to be located the computer countsthrough the end-of-field markers to locate individual

records and fields.

Variable Length Records


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Capturing, Entering and Editing

Data

Keyboard, barcode readers

Designing a data capture form

entry options for fields

data automatically entered into a field

much quicker and more accurate to choose from alist, rather than having to type it in - a value list (orcombo box )

check box

radio buttons / option buttons


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Check Boxes


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Value List


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Data Validation

Type the data is in the correct format: e.g. a number in a numericfield.

Presencefield is not left empty. The user is asked to enter data if

it has not been entered. Lookup listOnly allows values to be entered from a pre-defined

list.

RangeOnly allows values to be entered from a certain range: e.g.from 1 to 10

Check digitThat numbers have been entered correctly. An extranumber (the check digit) is added and a calculation is made whichshows whether the numbers have all been entered correctly. used

to verify such things as product codes in shops. .

Is data sensible?


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Data Verification

Rechecking for mistakes

A second person checking Double entry using software to discover

mismatches

Is data accurate?


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Processing Data

Searching(querying data) - criteria for the search

SQL (structured query language) . SELECT ALLWHERE

age > 13 AND name = Smith

Joining conditions - AND,ORand NOT.

Sortingascending/descending

Importing: from another database,csv files

Graphs- data exported to other programs to creategraphs

Calculationsuse formulas to add etc


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The information stored in computer files must be keptup-to-date

When a new record needs to be added, it is inserted.

Records are deletedwhen they are no longer needed.

Records are amendedwhen the data in a fields needsto be altered.

Updating 1


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Master filemain fileis updated by comparing it with thetransaction filechanges made.

Transaction file- details of all the changes that need to bemade to a master file are collected for later processing.

Three generations of a master file are kept for backup

purposes.

Updating 2

Master Transaction

New

Master

New New

Master

Transaction


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Types of Databases

Flat-file databasesone table

Relational databases

More than one table - linked

Advantage smaller file sizes and faster access.

data need only be changed in one file if files are linked. Ifthey are not linked.. ? (what needs to be done?)
http://d/XML/glossary.xmlhttp://d/XML/glossary.xml


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Tables Linked in a Relational

Database


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File Access

File organization Random access file

records are not stored in any particular order

sequential file records are sorted into some kind of order

Accessing files Serial and sequentialfiles- accessed by going through each

record until the required record is found

direct access (random access)files, the required record is

found straight away (directly) without first going through theother records.

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