According to me this is a very small chapter but very basic stuffs are discused in here.So read through very carefully try to understand the terms and feel free to ask any doubts we will try to clarify doubts as far as we can.From the previous chapters you would have got a brief idea about what is a database which are the different database models etc.Here we are going to start on a Relational database.

A Relational Database Management System uses the relational model that is relationship between records are maintained in the table.The features of an RDBMS include an interactive query language,retrieval of related data from more than one tables,a DataDictionary or catalog for easy access of data etc…

Some Important Terms with respect to DBMS

1. TUPLE: a row in a table is called a tuple.
2. RELATION: a table is usualy referred to as a relation.
3. ATTRIBUTES: a column in a table.
4. DEGREE: is number of attributes.
5. DOMAIN: a set of values from which attributes can take values.

Understanding the TERMS

Consider the same employee example i mentioned in the first chapter.While creating a (relation)table for the employees,the NAME,EMPLOYEE ID,DOB,DEPARTMENT,ADDRESS etc are attributes(columns),all these together form a tuple(row).Consider the attribute EMPLOYEE ID ,it takes the value as 6 digit positive integers,so that is the DOMAIN.Hope it is clear now.

KEYS : is an important topic that all of you should read properly.I will explain it with examples rather than with definitions.For this am using the same old example of an EMPLOYEE.I am going to list down a few ATTRIBUTES of it.

-Employee Name

-Employee ID(Eid)

-Domain

-Department

-DOB

-Blood Group

-Manager to whom he is reporting(Manager)

-CUBICLE ID(desk id/seat number)

-CUBICLE PHONE NUMBER(desk phone)

-Email Id

-Date of Joining(DOJ)

Certain attributes alone or in combination can distinguish one employee from another.Like Eid,Email Id both can itself identify an employee.Combination of EmployeeName+DOB+DOJ,EmployeeName+Manager+CUBICLE ID can also identify an employee.There can be many more combinations.From these we can simply take Eid as a unique identifier as it is the simplest key.

DEFINITIONS:-

-There may be 2 or more attributes or there combinations which can uniquely identify a record.These attributes or combinations are called CANDIDATE KEYS.Eg:-(Eid,Email Id,CUBICLE ID+CUBICLE PHONE).These are candidates for the primary key.Rest other than primary key in this set are called ALTERNATE KEYS.

-An attribute or set of attributes whose values uniquely identify each entity in an entity set is called KEY.

-If we add additional attributes to a key and it STILL uniquely identify an instance of the entity set such keys are called SUPER KEYS.

-A minimum super key is called PRIMARY KEY,it cant be NULL.

-A UNIQUE key can be NULL.

-Attribute or group which refers to some other table is called a FORIEGN KEY.

Any key consisting of a single attribute is called a SIMPLE KEY.And those with a combination of attributes are called COMPOSITE KEYS.With this i am winding this chapter.

Now that we have finished with the evolution of databases and the uses of a DB,the next important portion is understanding the architecture of a database system.A Db is complex piece of software that usually consists of a number of modules.As we know the PRIME aim of a DB is to handle data efficiently for the wide array of users,the DB must be aware of the data it is holding,or simply to help users, DB must maintain ” information about the DATA ” itself.This information include what data is stored,how it is stored,who can access to what parts of it etc.This information(DATA)about the data in a database is called the METADATA.

Users of a Database:A DB has three broad class of users-

1. Application Programmers.
2. End Users.
3. Database Administrator(DBA)#.

Application programmers are those people writing programs that use the database.End users are people who interact with the DB using an application or some interfaces,they usually dont know even about the existence of such software called database.DBA is the person who administers the DB.He creates the actual DB and implements the technical controls,maintains data security etc.

Components of A Database

1. Query Language:-is a programming language which is english like and more understandable,which helps the users to store and retrieve data.
2. Query Processor:-Translates statements in a query language into low level instructions that the DB understands.
3. Database Manager:- acts as an interface between applications programs,low level data and queries submited by users.
4. File Manager:-manages the disk space,storage and diferent data structures used to represent information on disk.

One basic characteristic of this database approach is that it provides a level of data abstraction by hiding certain details from users,which means each user will see only those data which they are expected to see.A DATA MODEL helps the DB in doing this.There are THREE categories of data models

1. Conceptual.
2. Physical.
3. Representational.

The goal of three schema architecture is to seperate the users from the physical database.The internal level is the one closest to physical storage,external is the one closest to the users,it is concerne about how data is viewed by diferent users.The conceptual level is an intermediate level between the other two levels.

A term while talking about databases that we come across is SCHEMA.What is a SCHEMA??Very precisely it is a description of data in terms of a data model.And the description of a database is called database SCHEMA.Now we are slowly moving to Database Models,

1. Relational Model:-Here every item in the DB is viewed as a record with certain attributes.And records with similar attributes is called a table.Eg: for DBMS’ which use relational model are Oracle,MySQL,Sybase etc.
2. Network Model:-This also considers data as records but difference from reational model is that each record will have a link field which will link to other records.Eg:-IDS(Integrated Data Store).
3. Hierarchical Model:-Here data is represented in hierarchical format as in a tree.This is a special type of network model,where a parent can have many children.

With this i am concluding on the initial requirements for studying a particular Relational Database Management System.From next chapter onwards we will give more stress to a particular DB,mostly Oracle or MS SQL Server.

#Functions of a DBA:

• Defining conceptual models and schema.
• Physical database design and mapping between internal and conceptual schemas.
• Defining security and integrity  rules.
• Defining backup and recovery mechanisms.
• Monitoring performance.

Introduction to Database Management Systems:

What is a database and how it emerged when a strong and widely accepted languges like C was there???The answer is simple in language like C the programmer deal with user and DATA directly where as in DBMS the programmer is least bothered of user.Only thing he need to take care of is effective management of  lumbs of data.Here i will try to explain DBMS and RDBMS mostly relating to ‘C’ assuming it as a known language to most people.The first thing that comes to our mind when we say a programming language is ofcourse programs.QUERY is the analogy in a DBMS.Now think about a situation where you have a program to store details of employees of a company,and DB is unknown to you.Then your only option for STORAGE and RETIEVAL of data is files.Any programmer knows the difficulties with files(FLAT files).Say we started using files,we have to store NAME,AGE,QUALIFICATION,EID,SALARY,DEPT,MANAGERID etc.of around 100 employees.Storing might be easy .But HOW about RETRIEVAL????? For retrieval we must store data in a particular fashion so that we ourself has to device a logic,like seperating each employee’s details with some special character,but again if we want any one of employee attributes again those also have to seperated somehow.All these will result in tedious program logic.THIS is HOW DBMS came in.

WHAT is a DBMS and its GOALS??

• Is a computerised record keeping system.
• Which can handle massive amounts of data.
• It provides for STORAGE ,RETRIEVAL,UPDATION of data in an organised manner.

GOALS

• Enforce security of data.
• Concurrency control and Crash recovery.

I would like to elaborate on the points in goals of a DBMS.Consider the case i mentioned earlier,a company and its employees.Who should have access to such a sensitive data?? Senior people only right?or else anyone can change critical details like salary.That is security of data.A DBMS will allow only authorised personnel.This is done using password,encryption etc.But every employee should have the right to check his data.Consider a scene when manager is editing the salary column of an employee,at that time employee is trying to edit his famiy details.Chances are there for an error,but it wont happen because DBMS provides CONCURRENT ACCESS CONTROL.

Def: of a Database Management System:- A DBMS is a collection of programs that facilitates the processof defining,constructing and manipulating databases.Defining a DB means specifying types of data,constructing means storing data and manipulation means updating and querying the Db.

Before finishing the first chapter i have one more relevant point to mention regarding,how a DB helps you.This is a very key point.Think about the same scenerio, the EMPLOYEE.We started without a DB,and used the Flat file system for some time.Now the company made a policy change,that it is going to give all its empoyees a special add on towards salary,which is not taxable,so it needs to be mentioned seperately with each employee.What will you do???With a DBMS the task is simple,this is called logical data independance.