1.Introduction to DBMS

Introduction
Data:-
Data is a collection of facts, figures and statistics related to an object. Data can be processed to create useful information. It also enables the user to make better decision for future. Data is very useful for generating reports, graphs and statistics.
Example:
Students fill an admission form when they get admission in college. The form consists of raw facts about the students. These raw facts are student’s name, father name, address, etc. The purpose of collecting this data is to maintain the records of the students during their study period in the college.

Data base:-
A data base is a collection of information that is organization so that is can easily be accessed, managed, and updated. In one view, database can be classified according to types of content; bibliographic, full-text, numeric, and images

Database Management System:
A database management system (DBMS), sometime just called a database manger, is a program that lets one or more computer users create and access data in a database.

Or
Database Management System (DBMS) is a software package that allows data to be effectively stored, retrieved and manipulated and the data stores in a DBMS package can be accessed by multiple users and by multiple application programs like (SQL Server, Oracle, and Ms-Access).
Or
A database management system (DBMS), sometimes just called a database manager, is a program that lets one or more computer users create and access data in a database. The DBMS manages user requests (and requests from other programs) so that users and other programs are free from having to understand where the data is physically located on storage media and, in a multi-user system, who else may also be accessing the data. The most typical DBMS is a relational database management system (RDBMS). A standard user and program interface is the Structured Query Language (SQL). A newer kind of DBMS is the object-oriented database management system (ODBMS). A DBMS can be thought of as a file manager that manages data in databases rather than files in file systems. Some DBMS examples include MySQL, PostgreSQL, Microsoft Access, SQL Server, FileMaker, Oracle, RDBMS, dBASE, Clipper, and FoxPro.


Application development without a database
Most programming languages have to ability to create and store data in separate files. COBOL programs make extensive use of basic files. Today many applications are written using traditional files to store data. The basics problems with file approach are :-
a. Uncontrolled duplication (Wastes space, hard to update all files), b. Inconsistent data, c. Inflexibility (Hard to change data), d. Limited data sharing, e. Poor programmer productively.
For example:-
If an application needs customer data, the program must define a customer files and contain all the code to create the files, store the data, and retrieve the data as it is needed and to store data in any format. It is possible to share to across application if all programmers follow the standards.



Advantages of DBMS:
1. DBMS Provides "Reduced Application Development Time".
No doubt it takes more time for designing a database than making a single application but once a database is created then it becomes very easy to make new application by using facilities provided by DBMS.
2. DBMS provides flexibility.
To the requirements we have to do changes in database. Using DBMS, we can change the structure of database and advantage is that it will not affect on the existing database.
3. Restricting Unauthorized Access
A DBMS provides a security and authorization subsystem, which is used by DBA to create user accounts and to specify restrictions on user accounts. File processing system provides password mechanism and very less security which is not sufficient to enforce security policies like DBMS.
4. Providing Backup & Recovery
Data should be restored to a consistent state at the time system crash and changes being made. If hardware or software fails in the middle of the update program, the recovery subsystem of DBMS ensures that update program is resumed at the point of failure.
5. Multiple user interfaces
DBMS provides a variety of user interfaces for the users of varying level of technical knowledge. These includes query language for casual users, programming language interfaces for application programmers, forms and command codes for parametric users, menu driven interfaces and natural language interfaces for stand alone users etc
6. Enforcing Integrity Constraints
The DBMS have certain integrity constraints that hold on data. These constraints are derived from the meaning of the data and of the miniworld. Some constraints can be specified to the DBMS at the time of defining data definitions and automatically enforced. Database does not allow violation of constraints at the time of updating the database.
7. Data Redundancy and inconsistency control in file system
The same data is to stored multiple times. This redundancy leads to several problem duplication of effort, wastages of storage space and inconsistency in data i.e. files that represent the same data show different value, this may happen because an update is applied to some of the files but not to others.
8. Representing Complex Relationships among data
A DBMS must have the capability to represent a variety of complex relationship among the data, to define new relationships as they arise, and to retrieve and update the related data easily and efficiently.


Disadvantages of DBMS
A database system generally provides on-line access to the database for many users. In contrast, a conventional system is often designed to meet a specific need and therefore generally provides access to only a small number of users. Because of the larger number of users accessing the data when a database is used, the enterprise may involve additional risks as compared to a conventional data processing system in the following areas.

Confidentiality, Privacy and Security
When information is centralized and is made available to users from remote locations, the possibilities of abuse are often more than in a conventional data processing system. To reduce the chances of unauthorized users accessing sensitive information, it is necessary to take technical, administrative and, possibly, legal measures.
Most databases store valuable information that must be protected against deliberate trespass and destruction.

Data Quality
Since the database is accessible to users remotely, adequate controls are needed to control users updating data and to control data quality. With increased number of users accessing data directly, there are enormous opportunities for users to damage the data. Unless there are suitable controls, the data quality may be compromised.

Data Integrity
Since a large number of users could be using a database concurrently, technical safeguards are necessary to ensure that the data remain correct during operation. The main threat to data integrity comes from several different users attempting to update the same data at the same time. The database therefore needs to be protected against inadvertent changes by the users.

Enterprise Vulnerability
Centralizing all data of an enterprise in one database may mean that the database becomes an indispensable resource. The survival of the enterprise may depend on reliable information being available from its database. The enterprise therefore becomes vulnerable to the destruction of the database or to unauthorized modification of the database.

Database Damage
In most of the organizations, all data is integrated into a single database. If database is damaged due to electric failure or database is corrupted on the storage media, then your valuable data may be lost forever.

Appointing Technical Staff
The trained technical persons such as database administrator, application programmers, data entry operators etc. Are required to handle the DBMS. You have to pay handsome salaries to these persons. Therefore, the" system cost increases.


Components of a DBMS
Database Engine
The database engine is the heart of the DBMS. It is the core service for storing, processing, and securing data. The Database Engine provides controlled access and rapid transaction processing to meet the requirements of the most demanding data consuming applications within your enterprise. This includes creating tables for storing data, and database objects such as indexes, views, and stored procedures for viewing, managing, and securing data.

Data dictionary:
The data dictionary stores the definition of all of the data tables. A data dictionary is a reserved space within a database which is used to store information about the database itself. A data dictionary is a set of table and views which can only be read and never altered. Most data dictionaries contain different information about the data used in the enterprise.

In terms of the database representation of the data, the data table defines all schema objects including views, tables, clusters, indexes, sequences, synonyms, procedures, packages, functions, triggers and many more.

A data dictionary is used when finding information about users, objects, schema and storage structures. Every time a data definition language (DDL) statement is issued, the data dictionary becomes modified.

Query Processor:
Query processors a fundamental component of the DBMS. It enables developers and user to store and retrieve data. It manages records, controls concurrency, and maintains log files. The query processor accepts SQL syntax, selects a plan for executing the syntax, and then executes the chosen plan. The user or program interacts with the query processor, and the query processor in turn interacts with the storage engine.

Form Generator
A form generator helps the developer to create input forms. The forms are used for data entry it enables developers to build forms by dragging and dropping item on the screen.

Application Generator
Application Generation Subsystem contains facilities to help users to develop transaction-intensive applications. It usually requires that user perform a detailed series of tasks to process a transaction. It facilitates easy-to-use data entry screens, programming languages, and interfaces.
An application is a collection of form and report design for a specific user task. A good DBMS contains an application generator which consists of tools that assist the developer in creating a complete application package.

Communication and Integration Utilities
These utilities are used to store and use data in several database running on different machines even if they are in different location.

Report writer:
Most report writers allow you to select records that meet certain conditions and to display selected fields in rows and columns. You can also format data into pie charts, bar charts, and other diagrams. Once you have created a format for a report, you can save the format specifications in a file and continue reusing it for new data.

Also called a report generator, a program, usually part of a database management system that extracts information from one or more files and presents the information in a specified format.

Security & other utilities
A primary goal of a database is store data with multiple users hence the DBMS must be responsible for establishing and maintaining security access controls. This unit is responsible for security issue. This describes now individuals or groups of users can be granted. Specific privileges and how their action can be restricted too specific areas of the database. The other utilities may include backup and recovery, user management, data storage evaluation, and performance monitoring tools.

Data Definition Subsystem helps user to create and maintain the data dictionary and define the structure of the files in a database.

Data Administration Subsystem helps users to manage the overall database environment by providing facilities for backup and recovery, security management, query optimization, concurrency control, and change management.

Functions of a DBMS
a. Data Storage, Retrieval, and Update.
b. A User-Accessible Catalog.
c. Transaction Support.
d. Concurrency Control Services.
e. Recovery Services.
f. Authorization Services.
g. Support for Data Communication.
h. Integrity Services.
i. Services to Promote Data Independence.
j. Utility Services.

Classification of DBMS
1. According to data model:-
a. Relational Model:- The relational model uses a collection of tables to represent both data and the relationship among these data. Each table has multiple columns and each column has a unique name.
b. ER Model(Entity Relationship):- The ER data model is based on a perception of a real world that consists of a collection of basic objects, called entities and of relationship among these object. An entity is a thing or objects in a real world that is distinguishable other object. It is a graphical view of entity and relationship.
c. Object Oriented Model:-The object based model is an extension to the relation model to incorporate object database concept and concept of encapsulation, method and object identity.

2. Based on Number of user supported by the system
a. Single User System:- Single User system supports only one users at a time and mostly used with personal computer.
b. Multi-user system:- Multi-User system supports multi users concurrently. It is mostly used in network environment.
3. According to the number of sites over which the database is distributed
a. Centralized DBMS:-In centralized database system, the data is stored at a single computer. It support multiple users but the DBMS and the data base themselves reside totally at a single computer site.
b. Distributed DBMS:- A distributed DBMS can have the actual database DBMS software distributed over many sites, connected by computer network.
4. According to the purpose
a. General Purpose DBMS:- These DBMS software are designed and built to use for general purpose. These can be used for management of data from any field and areas.
b. Specific purpose DBMS:- When performance is primary consideration, a specific purpose DBMS can be designed and built for the specific application. Such system can not be used for other application without modification.