Databases - BH
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Transcript of 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.