CSIT 2nd year 4th semester – DBMS notes (Part I)

CSIT 2nd year 4th semester – DBMS notes (Part I)

What do you mean by Data and Database?

Data can be divided into three categories.

Raw data – this could be “85” – doesn’t have meaning when it stands alone. It might mean

something if you knew it was weight of a man in Kilograms.

Related raw data is a group (data set or data file) of organized raw data that can be tied

together. For example, it could be a group of Names, weights, blood group and identification

numbers, all tied to the Identity cards issued to patients at hospitals

Cleaned raw data is all the above after being validated or processed through some process.

Such a process might ensure that blood groups doesn’t have any value as “red” or “black” for

example only allowed values could be of the kind A,A+,B,B+ etc.

Data can be acquired from many different sources. It must always be evaluated as to which

category it belongs, and if it needs any additional validation before analysis that produces

information.

Database:

A database consists of an organized collection of interrelated data for one or more uses,

typically in digital form.

Examples of databases could be: Database for Educational Institute or a Bank, Library, Railway

Reservation system etc.

What Is a DBMS?

Consists of two things- a Database and a set of programs.

1     Database is a very large, integrated collection of data.

2     The set of programs are used to Access and Process the database.

3     So DBMS can be defined as the software package designed to store and manage or process the  database.

 Management of data involves

o Definition of structures for the storage of information

o Methods to manipulate information

o Safety of the information stored despite system crashes.

 Database models real-world enterprise by entities and relationships.

o Entities (e.g., students, courses, class, subject)

o Relationships (e.g., Arjun studies in Class -EEE VII)

File System

? Data is stored in Different Files in forms of Records

? The programs are written time to time as per the requirement to manipulate the data

within files.

o A program to debit and credit an account

o A program to find the balance of an account

o A program to generate monthly statements

Disadvantages of File system over DBMS

Most explicit and major disadvantages of file system when compared to database management

system are as follows:

? Data Redundancy- The files are created in the file system as and when required by an

enterprise over its growth path. So in that case the repetition of information about an

entity cannot be avoided

Eg. The addresses of customers will be present in the file maintaining information

about customers holding savings account and also the address of the customers will be

present in file maintaining the current account. Even when same customer have a saving

account and current account his address will be present at two places.

? Data Inconsistency: Data redundancy leads to greater problem than just wasting the

storage i.e. it may lead to inconsistent data. Same data which has been repeated at several

places may not match after it has been updated at some places.

For example: Suppose the customer requests to change the address for his account in

the Bank and the Program is executed to update the saving bank account file only but his

current bank account file is not updated. Afterwards the addresses of the same customer

present in saving bank account file and current bank account file will not match.

Moreover there will be no way to find out which address is latest out of these two.

? Difficulty in Accessing Data: For generating ad hoc reports the programs will not already

be present and only options present will to write a new program to generate requested

report or to work manually. This is going to take impractical time and will be more

expensive.

 For example: Suppose all of sudden the administrator gets a request to generate a list

of all the customers holding the saving banks account who lives in particular locality of

the city. Administrator will not have any program already written to generate that list but

say he has a program which can generate a list of all the customers holding the savings

account. Then he can either provide the information by going thru the list manually to

select the customers living in the particular locality or he can write a new program to

generate the new list. Both of these ways will take large time which would generally be

impractical.

?Data Isolation: Since the data files are created at different times and supposedly by

different people the structures of different files generally will not match. The data will be

scattered in different files for a particular entity. So it will be difficult to obtain

appropriate data.

For example: Suppose the Address in Saving Account file have fields: Add line1, Add

line2, City, State, Pin while the fields in address of Current account are: House     No.,                    

Street No., Locality, City, State, Pin. Administrator is asked to provide the list of

customers living in a particular locality. Providing consolidated list of all the customers

will require looking in both files. But they both have different way of storing the address.

Writing a program to generate such a list will be difficult.

?Integrity Problems: All the consistency constraints have to be applied to database through

appropriate checks in the coded programs. This is very difficult when number such

constraint is very large.

For example: An account should not have balance less than Rs. 500. To enforce this

constraint appropriate check should be added in the program which add a record and the

program which withdraw from an account. Suppose later on this amount limit is

increased then all those check should be updated to avoid inconsistency. These time to

time changes in the programs will be great headache for the administrator.

?Security and access control: Database should be protected from unauthorized users.

Every user should not be allowed to access every data. Since application programs are

added to the system

For example: The Payroll Personnel in a bank should not be allowed to access

accounts information of the customers.

?Concurrency Problems: When more than one users are allowed to process the database.

If in that environment two or more users try to update a shared data element at about the

same time then it may result into inconsistent data.

For example: Suppose Balance of an account is Rs. 500. And User A and B try to

withdraw Rs 100 and Rs 50 respectively at almost the same time using the Update

process.

Update:

1. Read the balance amount.

2. Subtract the withdrawn amount from balance.

3. Write updated Balance value.

Suppose A performs Step 1 and 2 on the balance amount i.e it reads 500 and subtract 100

from it. But at the same time B withdraws Rs 50 and he performs the Update process and

he also reads the balance as 500 subtract 50 and writes back 450. User A will also write

his updated Balance amount as 400. They may update the Balance value in any order

depending on various reasons concerning to system being used by both of the users. So

finally the balance will be either equal to 400 or 450. Both of these values are wrong for

the updated balance and so now the balance amount is having inconsistent value forever.