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Introduction to DBMS

DATA BASE SYSTEM

Data Base System is a computer based record keeping system whose overall purpose is to record and maintain information. The information can be anything, that is of significance to the organization, the system is serving. ie. anything that may be necessary for the decision making process involved in the management of the organization.

A data base system has four major components – data, hardware, software and users.


DATA Data stored in the DBS may be partitioned into one or more database. A database is a shared collection of inter-related data designed to meet multiple needs of different types of end users (ordinary user). The data are stored so that they are independent of the programs that use them. A common and controlled approach is used in adding new data and modifying and retrieving existing data. The data in a database is both shared and integrated. By integrated, we mean that the data base may be considered as a unification of several otherwise distinct data files with any redundancy (repetitions) among those files partially or wholly eliminated. By shared, we mean that individual piece of data in the database may be shared among several users, in the sense that each of these users may have access to the same piece of data. Concurrent sharing of data is also possible that is, the ability for the several different users to access the same piece of data at the same time. A database system supporting concurrent sharing is some times referred to as multi user system.


HARD WARE
The hardware consists of the secondary storage volumes like disks, drums etc. on which the data base resides, together with the associated devices, control units, channels etc.

SOFTWARE Between the physical database and users of the system, is a layer of software usually called as database management system. All requests from the user for access to the database are handled by the DBMS. One general function provided by the DBMS is shielding of database users from the hardware level details. The other functions of the DBMS are data base definition, database creation (storing data in a defined database), retrieval (queries and reports), updation (changing the content of the database), providing facilities for database application program development, database revision and restructuring, database integrity control, performance monitoring.


USERS
Three broad classes of users are - end user, application programmer and database administrator (DBA). DBA is the person or group of persons who has overall control over the database system. Application programmers are responsible for writing application programs that use the data. The application programs may operate on the data in different ways mainly retrieving information, creating new information, deleting or changing existing information. All these functions are performed by issuing appropriate requests to the DBMS. The end user access the database by using a query language provided as an integral part of the system or may invoke an application program that accepts commands from the terminal and issue request to the DBMS.


ADVANTAGE OF DATABASE SYSTEM

The data base system provides Centralized control of its operational data. It has the following advantages :-

1.Redundancy can be reduced. In the non-database system each application has its own private files. This can often lead to considerable redundancy in stored data, which results in wastage of storage space.

Example : The personnel application and training application may each own a file containing department information of employees. In DBS (Data base system) these two files can be integrated and redundancy can be minimized or eliminated.

2. Inconsistency can be avoided . Suppose that employee E1 works in a department D1 is represented by two distinct entities in the database and that the system is unaware of this duplication. Then there will be some occasions on which the two entities will not agree ( i.e. when only one is updated ) . At such time, the database is said to be inconsistent .The database that is in an inconsistent state is capable of supplying incorrect or conflicting information. A data base is said to be inconsistent if two fields ( attributes ) which holds the same pieces of information has different values in them. If the fact is represented by a single entity, then inconsistency can be avoided. i.e. by the removal of redundancy. If the redundancy is not removed but controlled ( by making it known to the system) then the system could guarantee that the database is never inconsistent. This can be done by ensuring that any change made to either of the two entities is automatically made to the other. This process is known as propagating update.

3. Data can be shared. The data in a database can be shared by different users. Existing application can share the data. New applications can be developed to operate on the same stored data i.e., data requirement of new applications may be satisfied without creating any new stored file.

4. Standards can be ensured. With the Centralized control of the data base, the DBA can ensure that all applicable standards are followed in the representation of data. These standards may be any or all of the following - company, installation, departmental, industrial , national and international standard. Standardizing stored data formats is desirable for data interchange or data migration between systems.

5. Security restrictions can be applied. Since the DBA has complete control over the operational data, the DBA can ensure that the only means of access to the data base is through the proper channel. The DBA can define Authorization checks to be carried out whenever access to the sensitive data is attempted.

6. Integrity can be maintained. The problem of integrity is the problem of ensuring that the data in the data base is accurate. The data values stored in the database must satisfy certain types of consistency constraints. Inconsistency between two entries representing the same facts is an example for lack of integrity. Even if the redundancy is eliminated, the database may still contain incorrect data. For example, an employee may be shown as having worked 250 hours in a week. Centralized control of the database help in avoiding these situations by permitting the DBA to define validation procedures to be carried out whenever any update operation is attempted.

7. Conflicting requirements can be balanced. Knowing the overall requirement of the organization. The DBA can structure the database system to provide an overall service that is best for the organisation. For example, a representation can be chosen for the data in storage, that gives fast access for the most important applications, at the cost of poor performance in some other applications.

8. Integration of data. In a database, data are organised into a single logical structure with logical relationship defined between associated data entities. Users can easily relate one item of data to another related item.

9. Ease of application development. Cost and time for developing new business applications are greatly reduced since designing, building, and maintaining the tables (master files) are done by the DBA.

10. Reduced program maintenance. Program maintenance refers to modifying or rewriting old programs to make them conform to new data formats and access methods. Since the data base system have data independence, the Program maintenance is greatly reduced.

11. Data independence. (Definition) + In traditional system, application programs are dependent on their data files . The descriptions of the data and the logic for accessing those data are built into each individual applications program. Any change to the structure to the data file requires modifying or rewriting the application program Data independence is the major objective of the data base system.

DATA ABSRACTION

The major purpose of the DBS is to provide an abstract view of the data. The system hide certain details of how the data is stored and maintained. In order for the system to be usable the data should be retrieved efficiently. For this, complex data structures are required for representing the data in the database. The complexity is hidden from the users through different levels of abstraction, to simplify their interaction with the system. The three levels of abstraction are:

1. Physical level / Internal Level

2. Conceptual level

3. View level / External Level



PHYSICAL LEVEL : This lowest level of abstraction describe how the data is actually stored. At the physical level complex lower level data structure are described in detail.

CONCEPTUAL LEVEL
: The next higher level of abstraction describes what data are actually stored in the data base and the relationship that exist among the data. The entire database is described by using some relatively simple structures . Although the implementation of these simple structures at the conceptual level may involve complex physical level structures the user of the conceptual level need not be aware of these. The conceptual level of abstraction is used by the DBA , who decides what information is to be kept in the database.

VIEW LEVEL :The highest level of abstraction describes only part of the entire data base. Because of the large size of the data base some complexity remains even after using simple structure at the conceptual level. Many users of the database system will not be concerned with all these information. But such users need only a part of the data base. To simplify their interaction with the system the view level of abstraction is defined. The system may provide many views of the database.



(e.g. In C you may declare a record as follows:

struct cust

{

int custcode;

char custname[30];

char custhous[30];

char custplac[30];

long custpin;

}

This defines a structure with five fields. Each field has a name type and size associated with it. At the physical level, the record can be described as a block of consecutive storage locations. At the conceptual level, each record is described by the type definition and the relationship among record types. At the view level, several views of the data are defined. For example some application may require custcode & custname field. )

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