DATA ABSTRACTION

A major purpose of a database system is to provide the user only that much information that is required by them. this means that the system does not disclose all the details of data, rather it hides certain details of how the data is stored and maintained. A good database system ensures easy, smooth and efficient data structure in such a way so that every type of database user : end user, application system analyst, and physical storage system analyst, is able to access its desired information efficiently.

Since the requirements of different users differ from one another,the complexity of the database is hidden from them, if needed, through several levels of abstraction is order to simplify their interaction with the system. the various levels of database implementation are being discussed in the following section

VARIOUS LEVELS OF DATABASE IMPLEMENTATION

A database is implemented through three general levels : internal, conceptual and external so as cater to the needs of its users.

1. INTERNAL LEVEL ( PHYSICAL LEVEL)

The lowest level of abstraction , the internal level, is the one closest to physical storage. This level is also sometimes termed as physical level. It describes how the data are actually stored on the storage medium. at this level, complex low-level data structures are described in details.

2. CONCEPTUAL LEVEL

This level of abstraction describes what data are actually stored in the database. It also describes the relationships existing among data. At this level, the database is described logically in terms of simple data structures. The users of this level are not concerned with how these logical data structures will be implemented at the physical level. Rather, they just are concerned about what information is to kept in the database.

3. EXTERNAL LEVEL ( VIEW LEVEL )

This is the level closest to the users and is concerned with the way in which the data are viewed by individual users. Most of the users of the database are not concerned with all the information contained in the database. Instead, they need only a part of the database relevant to them. For example, even though the bank database stores a lot many information, an account holder ( a user ) is interested only in his account details and not with the rest of the information stored in the database. to simply such users interaction with the system, this level of abstraction is defined. The system, thus provides many views for the same database. The following fig. illustrates the interrelationship among these three levels of abstraction.

CONCEPT OF DATA INDEPENDENCE

The ability to modify a scheme definition in one level without affecting a scheme definition in the next higher level is called data independence. the concept of data independence proves beneficial in such a context.

There are two levels of data independence : physical and logical

1. PHYSICAL DATA INDEPENDENCE

Physical data independence refers to the ability to modify the scheme followed at the physical level without affecting the scheme followed at the conceptual level. That is, the application programs remains the same even though the scheme at the physical level gets modified. Modifications at the physical level are occasionally necessary in order to improve performance of the system.

2. LOGICAL DATA INDEPENDENCE

Logical data independence refers to the ability to modify the conceptual scheme without causing any changes in the schemes followed at view levels. The logical data independence ensures that the application programs remains the same. Modifications at the conceptual level are necessary whenever logical structure of the database get altered because of some unavoidable reasons.

It is more difficult to achieve logical data independence than the physical data independence. The reason being that the application programs are heavily dependent on the logical structure of the database.

DATABASE CONCEPTS

INTRODUCTION

A database system is basically a computer based record  keeping system. the collection of the data, usually refered to as the database, contains information about one particular enterprise. it maintains any information that may be necessary to the decision making processes involved in the management  of that organization.

A database may also be defined as a collection of interrelated data stored together to serve multiple applications; the data is such stored so that it is independent of programs which use the data; a common and controlled approach is used in adding new data and in modifying and retrieving existing data within the database.The data is structured so as to provide a foundation for future application development.

Database system

The intention of a database is that the same collection of data should serve as many applications as possible. hence, a database is often conceived of as the repository of information needed for running certain functions in a corporation or orgainzation. such a database would permit not only the retrieval of data but also the continuous modification of data needed for control of operations. it may be possible to search the database to obtain answer to queries or information for planning purposes.

PURPOSE OF DATABASE

In a typical file-processing system, permanent records are stored in various files.A number of different application programs are written to extract records from and add records to the appropriate files.But this scheme has a number of major limitations and disadvantages, such as data redundancy, data inconsistancy, unsharable data, understandized data, insecure data, incorrect data etc. A database management system is answer to all these problems as it provides a centralized control of the data.

Let us consider some of the advantage provided by the database system and see how a database system overcomes the mentioned problems.

1. DATABASE REDUCE THE DATA REDUNDANCY TO A LARGE EXTENT

Data redundancy means duplication of data. Non database system maintain separate copy of data for each application. e.g., in a college , student records are maintain and the hostal also maintain records of all those students who live in hostal. Though the records for hosteler students are already being maintain by the college, the hostal keeps a separate copy of it. As far as everything goes well, the above mentioned file processing system works well. But duplication of data may lead to inconsistency or incorrect data  at times.

the database system do not maintain separate copies of the same data. rather all the data are kept at one place and all the application that require data refers to the centrally maintain database. Now , if any changes to be made to data, it will be made at just one place and te same changed information will be available to all the applications refering to it. Thus, redundancy gets controlled and so are the problems associated with it.

2. DATABASE CAN CONTROL DATA INCONSISTENCY TO A LARGE EXTENT

This is really a corollary of the previous point. when the redundancy is not controlled where may be occasions on which the two entries about the same data do not agree. at such time data base is said to be inconsistance. obviously, an inconsistance database will provide incorrect or conflicting information

By controlling redundancy, the inconsistancy is also control. Even if there is some redundancy retains in the database due to some technical reasons, the database management system ensures that any change made to either of the two entries is automatically made to the others. This process is known as propagating updates.

3. DATABASE FACILITATES SHARING OF THE DATA

Sharing of data means that individual pieces of data in the database may be shared among several different users, in the sense that each of those user may have access to the same piece of data and each of them may use it for different purpose.

The database management system makes sure that not only the existing application can share the data in the database, but also that new application can be develop to operate against that same stored data, or it can be said that the data requirement of new application may be satisfied without having to create any new stored file.

4. DATABASE ENFORCE STANDARDS

The database management system can ensure that all the data follow the applicable standard. There may be certain standard laid by the company or organization using the database. Or there may be certain industry standard that must be satisfied by the data. Similarly, there may be national or international standards. Standardizing stored data format is particularly desirable as an aid to data interchange or migration between systems.

5. DATABASE CAN ENSURE DATA SECURITY

The information stored inside a database is sometime of great value to a corporation. Therefore, it must be kept secure and private.

A database management system ensures data security and privacy by ensuring that the only means of access to the database is throw the proper channel and also by carrying out authorization checks whenever access to sensitive data is attempted.

COMPUTER AND ITS FUNCTIONAL COMPONENTS

WHAT IS COMPUTER ?

FULL FORM :- commonly operating machine particularly used for trade, education and research

Fig. Computer System

DEFINATION :- A  computer is an electronic device that can perform a variety of operations in accordance with a set of  instructions called program.

 Computer can access and process data millions of times faster than humans can. A computer can store data and information in its memory, process them and produce the desired results. Computers can do a lot of different tasks such as playing games, railway reservation, weather forecasting, error detection and controlling the flight of a space aircraft etc.

FUNCTIONING OF A COMPUTER

Before we start discussing the functioning of a computer, let us understand the way we function. To understand this, we will take some examples.

1.      Consider a case when your mother ask  you  to bring a cake and pastries from the market. You go to the market, purchase both the things and give it to your mother.

2.      You want to talk to one of your friends. His phone number is 123589654. So you dial up the number and speak to your friend.

In all, the examples certain thing(s) were needed to work upon, the actual work was then carried out accordingly and finally the result was obtained. In these examples undergoes three stages : the first stage (things to work upon) is called input stage ; the second ( the actual work being performed) is called process stage and the third stage ( the result) is called output stage.

FUNCTIONAL COMPONENTS OF A COMPUTER

Since a computer follows input-process-output cycle, the first stage is performed in computer by input unit, second stage is performed by its central processing unit and the third stage is performed by output unit. Thus the basic structure of a computer is as shown in fig.1

Fig.1 Basic Computer System

INPUT UNIT :-

The input unit is formed by the input devices attached to the computer. Examples of input devices and media are keyboard, mouse, magnetic ink character reader (MICR), optical mark reader (OCR), joystick etc.

The input unit is responsible for takinginput and converting it into computer understandable form ( binary code). Since a computer operates on electricity, itcan under stand only the language of electricity i.e. either ON or OFF or high voltage or low voltage.

CENTRAL PROCESSING UNIT :-

It is the control centre for a computer. It guides, direct and governs its performance. It is the brain of the computer. The CPU has two components which are responsible for different functions. These two components are control unit and Arithemetic Logic Unit.

ARITHMETIC LOGIC UNIT ( ALU ) :-

The ALU performs all the four arithmetical ( +, -,*,/ ) and some logical ( <,>,=, >=, <> ) operations. When two numbers  are required to be added , these numbers are sent from memory to ALU where addition take place and the result is put back in the memory .In the same way , other arithmetic operations are performed ( through ALU only ).

       For logical operations also, the numbers to be compared are sent from memory to ALU where the comparison take place  and the result is returned to the memory. The result of a logical operations is either TRUE or FALSE .These operations provide the capability of decision –making to the computer.

CONTROL UNIT:-

The control unit controls and guides the +interpretation, flow and manipulation of all data and information. The CU sends control signals until the required operation are done properly by ALU and memory. Another important function of CU is the program execution. The CU gets program instructions from memory and executes them one after the other. After getting the instructions from memory in CU, the instruction is decoded and interpreted.

  The control unit even controls the flow of data from input devices to memory and from memory to output devices

OUTPUT UNIT:-

 The output unit is formed by the output devices attached to the computer . The output coming from the CPU is in the form of electronic binary signals which needs conversion in some form which can be easily understood by human beings i.e. characters, graphicalor audio visual . This function of conversion is performed by output units. Some popular output devices are VDU ( Visual Display Unit ), printer, plotter, speech, synthesizer and coder etc.

THE MEMORY:-

If a computer has a brain ( CPU ), it must also have the faculty which we call memory .

The memory of the computer is more like a predefined working place, where it temporarily keeps information and data to facilitate its performance. When the task is performed, it clears its memory and memory space is then available for the next task to be performed. When the power is switched off, everything stored in the memory gets erased and cannot be recalled.