"Database System concepts And Architecture".
INTRODUCTION
Database and database systems are an essential components of everyday life in modern society, databases and database technology have a major impact on the growing use of computers. It is fair to say that database play a critical role in almost all areas where computer are used including bussiness,electronic commerce,engineering, Medicine, law,education and library science.
Definition:-
A database is a collection of related data by data it means a known facts that can be recorded and that have implicit meaning and addresses of a group of people.
->Database has the following implicit properties.
*Database represents some aspects of real world sometimes called the miniworld or the universal of discourage changes to miniworld are reflected in the system/database.
*A database is a reflected in the system /database of data with some inherent coherent collection of data with some inherent meaning a random assortment of data cannot correctly be reffered to as a database.
*A database is designed,build and populated with data for a specific purpose.It has an intended group of usres and some preconcieved applications in which these users are interested.
"Defining the database"
Involves specifying the data types structures and constraints of the data to be stored in this database.
"Constructing the database "
Is the process of storing the data on some storage medium that is controlled by the Dbms.
"Manipulating a database"
Includes functions such as querying the database to retrive specific data updating the database to reflect changes in the miniworld.
"Sharing a database"
Allow multiple users and programs to access the database simultaneously.
"Characterstics of the database Approach"
->the main characteristics of the database are self describing nature of a database systems.
->Insulation between programs and are data abstraction.
->support of multiple views of the data.
->Sharing of data and multiuser transaction processing.
*self describing nature of a database system:
A fundamental characteristics of the database approach is that the database system contains not only the database itself but also a complete definition or description of the database structure and constraints this definitions stored in the dbms catlog which contains information such as the structure of each file the type and storage format of each data items,and various constraints on the data. The information stored in the catalog is called metadata and it describe the structure of the primary database .
*Insulation between programs and data and data abstraction.
In traditional file processing the structure of data files is embedded in the application programs.so any changes to the structure of a file may require changing all programs,that access that file by constant dbms access programs do not require such changes in most case.the structure of data files is stored in the dbms catalog separately from the access programs it is called as program dat independence.
Example:- a file access program may be written in such a way that it can access only student records of the structure shown if we want to add another piece of data to each student record say the birthdate.the new structure of student records will be accessed and used.
*support of multiple views of dbms
A database typically has many users each of whom may required a different perspective or view of the database a view maybe a subset of the datsbase or it contains virtual data that is derived from the database files but is not only explicity stored.
Some users may not need to be aware of whether the data they refer to is stored or derived a multiuser dbms whose users have a variety of distinct applications must provide facilities for defining multiple views.
Example:-
One user of the database may be interested only in accessing and printing the transcript of each student a second user who is interested only in checking that the students have taken all the pre requests of each course for each which they register.
*sharing of data and multiusers transaction :-
A multiuser dbms as its name implies must allow multiusers to access the database at the same time this is essential if data for multiple applications is to be integrated and maintained in single database.
Example:-when several reservation clerks try to be assign by only one clerk at a time for assignment to a passenger these types of applications are generally called online transaction processing applications.
A fundamental role of multiusers dbms software is to ensure that concurrent transactions operate correctly and efficiently.
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