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WHAT IS DBMS AND DIFFERENT DATABASE MODEL

DBMS also known as Data Base Management System or database system is a system which stores data in a correct manner in a computer system. Database is referred as the collection of data. The data base system manages all information which may be necessary and important while decision making processes which are involved in management of a company which uses it.The database system is defined as: it is the collection of interrelated data stored in a computer which offers many applications. The data stored in the computer system does not depend on the programs use the data. Thus we do not need to modify the data for different-different programs which accesses this data. The database system provides us to add new data or to modify and retrieve the existing data. The data is structured so as it can provide a base of future application development.

The purpose of database system: a database system is a collection of data which is needed by an organization for data processing. The database keeps data accurate, private and protects it from being damaged or stolen by unauthorized persons. The database provides organized data which can be used by different application programs with their different data requirements.
Let us consider some of the advantages provided by database system.
  1. Database reduces the data redundancy: duplication of data is known as data redundancy. A database system does not maintain any separate or duplicate copy of the data. As for example, in a college, records of every student are maintained like their name, date of birth, father’s name and their address etc and if some of the student are using hotel to live, the hostel administration will also have the records of those students. Suppose the permanent address of a student has changed and he or she informed about it to the college administration to change his or her address but he or she forgot to inform to the hostel then that can arise a big problem because if hostel administration want to send a letter to that student’s family then that letter will not be received by that student family. If the college and hostel use a database system to store all student information then this problem will never arise.
  2. Database can control data inconsistency: data inconsistency arises due to data redundancy. Data inconsistency means if two different records are present for a single student in database then the data is called inconsistent and this state is known as data inconsistency. We can control data inconsistency by controlling data redundancy.
  3. Database provides sharing of data: in a database system each and every piece of data can be shared among many users and application programs. Any user can have access to the same piece of data for their particular use. Similarly every application program can access the data and those new application programs which will be developed in future will be able to access this data. Thus we do not need to create a new copy of the data for different user and applications.
  4. Database maintains standards: database management system maintains some standards laid by the company or industry which uses the database. There may also be some national and international standards of the data. Thus standardizing of data can be done only by the database system.
  5. Database provides data security: sometime data stored in a database system have great value thus database system provide security to data so that no unauthorized person can access the data. Data security refers to the protection of data against accidental or intentional disclosure to unauthorized person, or unauthorized modification or destruction. Privacy of datarefers to the rights of individuals and organizations to determine for themselves when, how, and to what extent information about them is to be transmitted to others.
  6. Integrity can be maintained through database.

TYPES OF DATABASE MODEL

There are three distinct data modes which are used for database management are:
  1. Relational data model
  2. Network data model
  3. Hierarchical data model

RELATIONAL DATA MODEL

In relation data model data is arranged into tables and these tables are termed as relations. The relational data model is based on a collection of tables.The row of a table is called tuple and column is called attributes. A row or tuple of a table or relation represents relationship among different columns or attributes. Each attribute has a unique name. And each tuple has a unique ID which is used to identify a particular row of a table. In relational data model we can query these tables, add a new tuple, delete a tuple and modify a tuple any time. A sample of relational data model is shown here.
Employ (E_code, E_name, City, Salary)

Company (C_code, C_name, city)

Works (E_code, C_code)

Underline field represent primary keys.

Some important points:
  1. Primary key: a primary key is a set of one or more attributes which can uniquely identify any tuple in a relation. Here for example e_code is the primary key.
  2. Candidate key: those attribute which can serve the purpose of primary key candidate key. For example in the employ table e_code and e_name are the two possible candidate keys. 
  3. Alternate key: in the set of two or more candidate keys, the keys which do not serve the purpose of primary key is called alternate key. For example in employ table e_name is the alternate key.
  4. Foreign key: foreign key is the key which shows relationship between two tables.
  5. Tuple: The rows of a table or relation are known as tuple.
  6. Attribute: The columns of a table or relation are known as attributes.
  7. Degree: The number of columns in a table or relation is called degree. A relation having n attributes has degree n.
  8. Cardinality: The number of rows in a relation is called cardinality. A relation having m rows has cardinality m.
  9. Data independence: The ability to modify a scheme definition in one level without affecting a scheme definition in the next level is called data independence. 
The above sample example is shown in relational data model as:
Employ Table
E_code E_name City Salary
100 Arun Kumar Kolkata 10000
101 Jai Gupta Delhi 20000
102 Abhishek Rathi Mumbai 15000
103 Gautam Gupta Varanasi 18000
104 Surya Shekhar Kolkata 25000
105 Kirti Roy Chennai 30000

Company Table
C_code
C_name
City
200
HSBC
Kolkata
201
CESC
Kolkata
202
ABCD
Howrah
203
RICE
Kolkata
204
SBI
Varanasi
205
RBI
Delhi
206
PNB
Punjab
207
IPL
Mumbai

Works Table
E_code
C_code
100
201
101
206
102
204
103
205
104
200