1

Chapter # 8

How Data is stored DATABASE

2

Items• An item of data means the smallest piece

of data that may be a single name or a single number.

• Field• A field is a area of a record reserved for

one particular type of data.• Each field contain one or more data

items.

3

Record• Record is a group of related items of data

which together can be treated as a unit.• The records are similar to one another in the

type of data.

• Files• A file is organized collection of related

records.• In computing a file is usually on backing store.

4

Examples

RollNo

Name Phone# DOB

1 Ali 042-78888 12/12/98

2 Ahmed 544545 09/4/97

FIELD

RECORDITEM

A Students Files

5

Reasons for using files• In any situations requiring a

permanent source of data.• In computer when the main store

is so small for amount of data being processed.

6

Key field• A key field is field that may be used to

search a file.• It is always useful to have one field

which is unique to a particular record.• Admission form no of each student.• With the library system, the key field

would be the number given to the book

7

Fixed and variable length fields

• Fields may be of fixed or variable length.• If the fields are of fixed length then a given

field in a record has a set number of characters positions reserved for it.

• We specify how long each field is going to be when we set up the structure of a database

• If the fields are of variable length, the number of characters in each is not determined beforehand.

8

Advantages of variable over fixed length

• There is no waste of storage space

• Advantages of fixed over variable length

• Computer operations such as searching can be carried out quickly.

• Allocation of storage space is more straight forward

• Updating the file is much easier

9

Types of Files• 1.Master file• Is a file used as a reference for a

particular computer application• It may be updated when necessary• It is the most complete and up-to date

version of a file.• If the master file is lost or damaged and

it is only copy, the whole system will break down.

10

Transaction file• Is a file of temporary data which has

been prepared in order to carry out a processing operation with the data on a master file

• It is used to update master file.• A transaction is a piece of a business.• Transaction can occur in any order, so

it is necessary to sort a transaction file into the same order as the master file

11

Backup or security files• Backup copies of files are in kept in case the

original file is damaged or lost or cannot be used

• The importance of master file, back up copies of it should be taken at regular intervals in case it is stolen, lost or corrupted.

• Looking after your disks is not enough you should always keep backup copies of all important data.

12

Transaction log files• Are bits of business such as

placing an order, updating the stock, making a payment.

13

File Access Methods• Serial access• Sequential access • Random Access

14

Serial access• To read a serial file ,a computer has to

read each record until it reaches the one required.

• The records do not follow each other in any particular order.

• So if another record needs to be added it can just be added to the end of the file.

15

Sequential Access• Access to a file is sequential when

records can only be written to or read from it in order one after another.

• So if just one record is required then method is slow but if all the records are required then it becomes fast

• Sequential files can be stored on • Magnetic tape

16

Direct or Random Access

• When any record can be accessed without having to access other records first

• It store data on direct access.• You can access both sequential

and random files directly

17

Advantages of direct access over sequential access

• Selected records can be accessed far more quickly from direct access files.

• Records can be accessed in any chosen order.

• Records do not have to be put into any particular order before the file is created

18

Advantages of Sequential access over direct access• Sequential files can be stored on

most media including magnetic tape ,paper tape and punched card

• For the programmer ; it is usually easier to write programs to handle sequential files

19

Factors for choice of method

• How may records are to accessed. If the records are to be accessed singly or few then direct access is used.

• How large the file is. If a file is small the time delay in searching it is not important therefore a sequential file is acceptable.

• Sequential files is often suitable for batch applications.

• The type of storage medium being used. If magnetic tape is to used then files may have sequentail.

20

Files Systems• Manual File System • Mechanical File System • Electronic File Systems

21

Manual File• File is Processed manually without

using any machine or tool.Problems:• Accessing the same information in

same of two people can not possible. • If a file is placed in wrong place then

finding it again can be problem • Updating the file is difficult.

22

Mechanical File System• File is processed by using different

device like typewriters, mechanical printers.

• That method is faster and more accurate than manual file.

23

Electronic file• File is processed through

computers• Files and set of instructions are

given to the computer as input and computer automatically processed that file.

24

Advantages of computerized system

• Information is stored only once, since most database software allows you to access information from several files.

• Files can be linked together which means that if you update one of the files.

• Changing is in easy way • Data loss is less chance • Validations checks may be made on the

data as it is being entered into database.

25

Disadvantages• If the computer breaks down you

are not able to access the details.• It is easy to copy computer file so

there is a need to protect sensitive data.

• Training is needed to use the system

26

Data base • Database is organized collection of

information or data.• Collection of related data stored in

an efficient and compact manner.

27

Examples• ATM• NADRA• Flight Details• Hotmail Or Yahoo Or Google• Library in Schools

28

Types of Database• Centralized database• Distributed database

29

Centralized database• Complete data is located at a

single computer and all the users access this single database.

• Its an efficient way to access and update data.

• Used in computer network environment.

30

Distributed database• Is a single database which is

spread physically across multiple computers over wide area network .

• It may be• Homogenous • Heterogeneous

31

DBMS• Database management system is

computerized record keeping system.

• Is a collection of programs, which are used to define ,create and maintain database.

32

Database System of a School

StudentsData

DBMS

Admission program

Schedule Program

33

Common File processes• File updating • Insertions• Deletions • Amendments

34

File Updating• Involves bringing a file up-to-date

with any changes that have occurred.

• There are various process that can take place during updating.

35

Insertions• A new entry in the file.• Suppose we had a file containing

the details of pupils in a school. • A new pupil may have joined the

school and so that pupil’s details will need to be added to the file.

36

Deletions• A delete record in the file.• A pupil could have moved to a

new school so that the pupil’s detail will need to be removed from the file.

37

Amendments• Some changes occur in the field of

file.• A pupil has moved house so the

address field needs changing.

38

File Manipulations• Sorting • Merging

39

Sorting • Some time a field need to be

sorted in particular order.• It may be • Ascending • Descending

40

Merging • Sometimes we want to combine the

contents of two files to form a single• The process of combining the files is

called merging.• If the two files to be merged are on

magnetic tape then we must sort the tapes into order before merging them.

• Merging is very important when we want to divide the work.