Chapter 8 How Data is stored DATABASE Mazharjaved

Chapter # 8 How Data is stored DATABASE Mazharjaved 2001@yahoo. com (0333 -4461420) 1

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 2

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 3

Examples FIELD Roll Name No 1 Ali 2 Ahmed A Students Files Phone# DOB 042 -78888 544545 12/12/98 09/4/97 RECORD Mazharjaved 2001@yahoo. com (0333 -4461420) ITEM 4

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 5

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 Mazharjaved 2001@yahoo. com (0333 -4461420) 6

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 7

Advantages of variable over fixed length • There is no waste of storage space • Advantages of fixed over vaiable 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 Mazharjaved 2001@yahoo. com (0333 -4461420) 8

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 9

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 Mazharjaved 2001@yahoo. com (0333 -4461420) 10

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 11

Transaction log files • Are bits of business such as placing an order, updating the stock, making a payment. Mazharjaved 2001@yahoo. com (0333 -4461420) 12

File Access Methods • Serial access • Sequential access • Random Access Mazharjaved 2001@yahoo. com (0333 -4461420) 13

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 14

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 • Magnetic disk Mazharjaved 2001@yahoo. com (0333 -4461420) 15

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 Mazharjaved 2001@yahoo. com (0333 -4461420) 16

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 Mazharjaved 2001@yahoo. com (0333 -4461420) 17

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 Mazharjaved 2001@yahoo. com (0333 -4461420) 18

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 19

Files Systems • Manual File System • Mechanical File System • Electronic File Systems Mazharjaved 2001@yahoo. com (0333 -4461420) 20

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 21

Mechanical File System • File is processed by using different device like typewriters, mechanical printers. • That method is faster and more accurate than manual file. Mazharjaved 2001@yahoo. com (0333 -4461420) 22

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 23

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 24

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 Mazharjaved 2001@yahoo. com (0333 -4461420) 25

Data base • Database is organized collection of information or data. • Collection of related data stored in an efficient and compact manner. Mazharjaved 2001@yahoo. com (0333 -4461420) 26

Examples • • • ATM NADRA Flight Details Hotmail Or Yahoo Or Google Library in Schools Mazharjaved 2001@yahoo. com (0333 -4461420) 27

Types of Database • Centralized database • Distributed database Mazharjaved 2001@yahoo. com (0333 -4461420) 28

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 29

Distributed database • Is a single database which is spread physically across multiple computers over wide area network. • It may be • Homogenous • Heterogeneous Mazharjaved 2001@yahoo. com (0333 -4461420) 30

DBMS • Database management system is computerized record keeping system. • Is a collection of programs, which are used to define , create and maintain database. Mazharjaved 2001@yahoo. com (0333 -4461420) 31

Database System of a School Students Data DBMS Mazharjaved 2001@yahoo. com (0333 -4461420) Admission program Schedule Program 32

Common File processes • • File updating Insertions Deletions Amendments Mazharjaved 2001@yahoo. com (0333 -4461420) 33

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 34

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 35

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 36

Amendments • Some changes occur in the field of file. • A pupil has moved house so the address field needs changing. Mazharjaved 2001@yahoo. com (0333 -4461420) 37

File Manipulations • Sorting • Merging Mazharjaved 2001@yahoo. com (0333 -4461420) 38

Sorting • Some time a field need to be sorted in particular order. • It may be • Ascending • Descending Mazharjaved 2001@yahoo. com (0333 -4461420) 39

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. Mazharjaved 2001@yahoo. com (0333 -4461420) 40