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.