ion and Structure of Data

8/2/2019 ion and Structure of Data

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Organisation and Structure

of Data


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of Data

Introduction

This unit is identified by a filename

A collection of data

stored in one unit

What is a file?


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File Structure

A file is made up of a number of records

A record is made up of a number of fields

Each line in a file is a

record

Each field in a record

holds a piece of data


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File Structure

RecordType

Credit /Debit

Date Description Amount

B C 01/05/2011Counter

credit

250.00

B D 03/05/2011Chq No:176534

26.34

B D 04/05/2011Chq No:176535

134.28

B C 12/05/2011Countercredit

50.00

Records

Fields


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File Structure

When all the records are the same lengththe file is said to be a:

fixed length file

ID Forename Surname DOB

92013 Sidney JONES 23/041978

92017

Lorraine LAIDLAW 12/11/1979

92114 Mandy MERRITT 08/03/1979

File structureID: 5Surname: 15Forename: 15

DOB: 8


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Fixed Length Files

Advantage: Reading them can be very fast because the computer

knows where each record/field is

Disadvantage: Lots of unused space in each record, therefore larger

file sizes


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File Structure

If each record has a different recordlength, then the file is said to be:

variable length file

Note how each field is separated by an *


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File Sizes

Can only be worked out on fixed length files

1. Need to work out how long each field is:

ID: 5Surname: 15Forename: 15YearGroup: 2FormNo: 1DOB: 8

2. Add the fields togetherto get the record size:

46

3. Finally multiply by thenumber of records inthe whole file:

46 x 1000 = 46,000 bytes


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File Structure

Fixed length files: Very good when you have lots of data that is

always the same length

Eg: transaction details (transaction id, accountnumber, bank sort code, amount)

Variable length files: Very good when you fields that differ in size Eg: customer details (title, name, address,

gender, DOB,etc)


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File Types

Serial file:

Records on this type of file will be readfrom top to bottom

Contains data in no particularorder records are usually stored

in the order they are received


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File Types

Sequential file:

File is still read from top to bottom, but issorted first

Records are stored in some sort

of order for example accountnumber order


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File Types

Indexed sequential file:

The file can be: Read sequentially from top to bottom

A specific record found because of the index

Each record is given a key which

uniquely identifies it, and the fileis kept in the key order


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File Types

Random access file:

Because of the key, the file does not needto be kept in order

The file will be accessed based on the key

and not reading from top to bottom

Each record is given a unique key

is generated by an algorithm


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Adding Data to Files

Serial files: New records are appended to the bottom of

the file

Sequential files: Records are read in

one at a time and thenew record is slottedin when appropriate

1 2 3 5

Old file

1 2 3 4 5

New file

4


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Adding Data to Files

Indexed sequential files: The record is added based on the key being

slotted into the correct place, the same as

sequential files

Random access files: The records dont need to be stored in any

order. The index that points to where therecord is located is updated


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Deleting Data from Files

Serial files: Each record is copied

across one by one to a

new file. The deletedrecord is just missedout

Sequential files: Same as with serial files

1 2 3 4 5

1 2 3 5

Old file

New file


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Deleting Data from Files Indexed sequential files:

Both the key and the record must be deleted

Random access files: As the record can be stored anywhere, only the

key needs to be deleted without reorganisingthe file structure


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File Types

All of the file types described previouslyare relatively old, though they are still used

A newer method for storing data is a:

database


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Databases

What is a database?

What do we mean by management of the data?

Selection (retrieval) of data Updating data already there Inserting new data Deleting data we dont want anymore

A collection of related data organised in astructured way so as to allow easy

management of the data


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Databases

What is data?

What is information?

Raw facts or figures

Data that has a meaning


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Databases

So, is this data or information?

20/06/1969 Data

Whilst it is obviously a date, you dont knowwhat the date relates to

Date of first moon landing = 20/06/1969

Now it is information


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Databases

A database is managed by:

A DBMS is:

Database management system (DBMS)

A collection of programs that provides thenecessary tools to create and manipulate the

data in a database


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Database Management System

The DBMS sits betweenthe applications you arerunning and the files

that are holding thedata

DBMS

Applications

File System

The DBMS will manage this data by: Checking for data inconsistencies

Minimising duplicated data

Retrieve related data from different files


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Databases

Advantages DisadvantagesData can be accessed quicklyand manipulated to createnew data

Requires time to set up

A single database can beshared by many users

Centralised data can beeasier to steal

Data validation can ensuregood quality data

If data is not correct then allusers will see the wrong data

Data duplication can beavoided

Security of data can be

centralised


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Elements of a Database

Table:

Record:

Field:

A complete set of data the equivalentof a file. Also known as an entity

One row of related data within a table

A property or characteristic of a table.Also known as an attribute


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Types of Database

Flat-file database

Relational database

A single table

Multiple tables


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Designing a Database

The first thing to do is to work out whatdata we have to store

To do this you would do the analysis part ofthe software development lifecycle

Questionnaires

Interviews

Any current documentation


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Designing a Database Next, we need to decide what is data and

what is information

21 dataAge:

information

How could we store this?

Date of birth


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Designing a Database Next, we need to break everything down

into:

Atomic data

Mr Brad Pitt

Could be heldas name But we have 3

separate

pieces of data

TitleForename

Surname

Atomic data


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Data Types

Standard data types:

String / TextInteger / Number / Short / LongDecimal / Real / Single / Double

Date and TimeBoolean

Why dont we have currency?


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Databases

To help organise the data we need a

This is

Primary key

A field in which a unique piece ofdata can be held for each record


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Primary Keys

What sort of field would be suitable for:

Students at college?

Customers for an online shop?

Cars at a garage?

StudentID

Email address / CustomerID

Car registration


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Getting Data Back

One of the advantages to using a database isthe ability to get back just the data you want

For example: All the names of students over 18

Registrations of cars with no road tax

How much tax a specific employee paid within acertain timeframe

We do this with the use of queries


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Queries

Queries involve using a special languagecalled

to get back and manipulate data from thedatabase

Structured Query Language (SQL)


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Queries

Queries can be used in a number of ways:

SELECTing records from the database thatmeet a specific criteria (filtering)

COUNTing the number of records that meet a

specific criteria

Performing calculations on fields that meet

certain criteria (eg: SUM)


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Validation and Verification

When a user enters data into a systemthere is a chance that it could be wrong

GIGO

Garbage In, Garbage Out


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Validation and Verification

Errors can occur

When data is captured

When hardcopy data is copied onto a computer

When data is transmitted within a computer system

When data is being processed by software

To prevent this we use validation andverification


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Verification - Validation

Checking bycomparison that noalterations have beenmade to data as it is

transferred from onesystem to another oron first entry onto acomputer

EG: keying data twice andcomparing input/outputs

Checking that data issensible rejectingdata that is not.

EG: presence check /format check.


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Transcription errors

Occurs when data in manually copied


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Usually due to :

Bad handwriting

Misreading

Mishearing

Long strings of meaningless numbers


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Main type - transpositional errors:

When two characters are swapped over

134638

136438


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Activity

For each of these transcription errorsexplain what is wrong and how the error islikely to have been made:

SO23 5RT entered as SO23 SRT Leeming entered as Lemming 419863 entered as 419683

2000000 entered as 200000 238.591 entered as 2385.91 23/05/89 entered as 23/05/07 199503 entered as 195503


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Transmission errors

Data that is already entered correctlyon a computer becomes corrupted whentransferred to another computer


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Processing Errors

Errors due to incorrectly written software

Could be due to: Incorrect calculations

Records being ignored

Wrong records being updated


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Verification

Remember this is

For example:

Double-entry verification Using check-digits

On-screen verification

Checking that data is correct


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Verification

Double-Entry Verification:

This is the entering of data twice

For example: Passwords

Email addresses


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Verification

On-screen verification:

This is where the user is presented with theentered data for them to manually check over

Often used in online forms when signing up to

things


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Verification

Check digit:

A digit in a numerical field used to check that

the overall sequence of numbers are valid

Done by automatically performing a calculationon the other numbers

EG: Barcodes

Bank account numbers


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Validation

Remember this is

For example:

Character check

Checking that data follows the rules of

the program and is sensible


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Validation

Character check:

Check for the appropriate range of

characters

For example: Upper / lower case characters Numeric / alpha-numeric in correct fields


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Validation

Format check:

Checks to see if data is in a valid format

For example: Dates being DD/MM/YYYY


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Validation Length check:

Checks to see the length of the data enteredis correct

For example: Account numbers Credit card numbers


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Validation

Range check:

Checks that data is neither too large or smalland fits within certain parameters

For example: Age is over 18 after DOB entered


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Validation

Look-up lists:

Only allowing the user to select values from alist

For example: Titles when entering name (Mr, Mrs, etc) Months when entering a date


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Validation Presence check:

Checks to see whether any data has beenentered into a field

For example: When filling out online forms


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Validation

Type check:

Checks to see if the appropriate data type hasbeen entered

For example: Checking that text isnt entered into a number field Checking that only dates are entered into

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