Definition:-A database management system (DBMS) consists of collection of data which are interrelated with each other and a set of programs used to access those data.
It is collection of data about one particular enterprise and it is designed to mange large information and creating an environment where we can retrieve and store database information.Data which is stored in database should be safe .This safety is managed by data base system.
Earlier one concept was there known as file processing.Its having various disadvantage which is solved in database management systems
DISADVANTAGE OF FILE PROCESSING:-
1.DATA REDUNDANCY AND INCONSISTENCY
In file processing same data is stored in different locations which leads data redundancy for this large storage area is required which increase the cost.Data redundancy means duplicate data stored in different location.
Now suppose you want to modify any data in case of file processing modification is very difficult because data may not modified at all the places in same time so this create huge problem which is known as inconsistency.
2.SECURITY PROBLEM
In file processing security is the main problem as data can be accessed by all users and modified by all users.In case of database system all the users are not applicable to access or modify the data. In case of database system users are classified based upon the different accessibility of different users .Higher level user or administrator accessibility is different from lower level user or normal user.
3.DIFFICULT TO ACCESS DATA
In File processing needed data is not retrieved in convenient and efficient manner which is not in the case of database system.
4.DATA ISOLATION
In case of file processing data is stored in different location and different formats so it is difficult to develop any program to access the stored data from different location ,in one words data is not isolated and in case of database data is isolated properly.
5.INTEGRITY PROBLEM
In case of file processing same data is stored in different location so to integrate the data developers are facing problem which is not in the case of database management system.
6. ATOMICITY PROBLEM
In any transaction suppose in case of ATM transaction a customer want to withdraw money here two conditions are there either customer gets money means successful transaction or not get the money due to system failure called unsuccessful transaction.A transaction is set of commands should be completely applied or not applied at all .This is called atomic.It is the main criteria for money exchange.It is difficult to ensure this property in file processing .
7.CONCURRENCY PROBLEM
In any application multiple user access the data and updates the same data simultaneously.There is a chance of inconsistent data updation so to overcome it some mechanism introduced in database system called concurrency control which mechanism not present in case of file processing.
Database management system overcome all the drawbacks of file processing system.
DATA INDEPENDENCE
Why we need database system ?
A major purpose is to abstract the view of data for end user means data are hides from the end users and how the data are stored and how the data maintained all this are hided in case of database management system.
In database system data which are stored should be maintained such that it should be retrieved efficiently ,design may be complex for this but user are not aware of it that is the user is unknown about the background of database system which is nothing but mechanism,complexity ,stored procedure,and maintenance all this is totally hidden for end user this is data abstraction.
In Database system three levels are followed
1.Physical Level
It is the lowest level of data abstraction
It describe how data are actually stored .
Database administrators only aware of the certain details of the data.
2.Logical Level
It is the next higher level after physical level.
It abstract what data are stored.
It also abstract the relationship among those data.
3.View level
It is the highest level of data abstraction .
It describes only part of entire database.
In a large application data are extracted from more than one file but user is unknown about this information .
DATA MODELS
Data model is nothing but collection of tool which describe data ,data relationship,data semantics and consistency.
Entity relationship model
An entity is nothing but collection of objects and entity relationship means relationships among these objects.Entities are set of attributes which are described form.A relationship is an association among different entities.
Relational model
It is collection of tables to represent both data and the relationship among those data.Each tables having multiple columns and all the columns having unique name
