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Introduction To DBMS

 Database Management System

 A database is an organized collection of data. A Database Management System is a collection of interrelated data and a set of programs to access those data. The main purpose of DBMS is to provide a way to store and retrieve database information that is both convenient and efficient. Management of data involves both defining structures for the storage of information and providing mechanisms for the manipulation of information. In addition, the database system must ensure safety crashes or attempts at unauthorized access.

Applications of Database System

  • Banking: For customer information, accounts, and loans, and banking transactions.
  • Airlines: For reservations and schedule information. Airlines were among the first to use databases in a geographically distributed manner. Terminals situated around the world accessed the central database system through phone lines and other data networks.
  • Credit card transactions: For purchase on credit cards and generation of monthly statements.
  • Telecommunication: For keeping records of calls made, generating monthly bills, maintaining balances of prepaid calling cards, and strong information about the communication networks.
  • Finance: For storing information about the holdings, sales, and purchases of financial instruments such as stocks and bonds.
  • Sales: For the customer, product, and purchase information.
  • Manufacturing: For management of supply chain and for tracking the production of items in factories, inventories of items in warehouse/stores, and orders for items.
  • Human resources: For information about employees, salaries payroll taxes and benefits, and for generation of paychecks.


Objectives of DBMS

  • Provide for mass storage of relevant data.
  • Make access to the data easy for the user.
  • Provide prompt response to user requests for data.
  • Make the latest modification to the database available immediately.
  • Eliminate redundant data
  • Allow for multiple users to be active one time.
  • Allow for growth in the database system
  • Protect the data from physical harm and unauthorized access.


Data Abstraction

For the system to be usable, it must retrieve data efficiently. The need for efficiency has led the designers to use complex data structures to represent data in the database.
Since many database system users are not computer trained, developers hide the complexity from the users through several levels of abstraction, to simplify user's interactions with the system. The different levels of data abstraction are as follows:

  • Physical level:   It is the lowest level of abstraction. It describes what data are stored in the database and what relationships exist among those data.
  • Logical Level:  The logical level thus describes the entire database in terms of a small number of relatively simple structures. Database administrators, who must decide what information to keep in the database, use the logical level of abstraction.
  • View Level:  The highest level of abstraction describes only part of the entire database. Even though the logical level uses simpler structures, complexity remains because of the variety of information stored in a large database. Many users of the database system do not need all this information; instead, they need to access only a part of the database. The view level of abstraction exists to simplify their interaction with the system. The system may provide many views for the same database.

Data Independence

It can be defined as the capacity to change the schema at one level of a database system without having to change the schema at the next higher level. There are mainly two types of Data Independence:
  1. Logical Data Independence: The ability to change the logical schema without changing the external schema(user view) is called logical data independence. For example, the addition or removal of new entities, attributes, or relationships to the conceptual schema should be possible without having to change existing external schemas or having to rewrite existing application programs.
  1.  Physical data independence: The ability to change the physical schema without changing the logical schema is called physical data independence. For example, a change to the internal schema, Such as using different file organization or storage structures, storage devices, or indexing strategy, should be possible without having to change conceptual or external schemas.


Instances and Schemas

Database change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an instance of a database. The overall design of the database is called the database schema. Schemas are changed infrequently if at all. The concept of database schemas and instances can be understood by analogy to a program written in a programming language 


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