levles

Q. Describe file system versus database management system / disadvantage of file system / purpose of data base management system.

Ans: Keeping organizational information in a file processing system has a number of major disadvantages:

• Data redundancy and inconsistency: Different programmers create the files and application programs over a long period, the various files are likely to have different structures and the programs may be written in several programming languages. The same information may be duplicated in several files. This redundancy leads to higher storage and access cost. It leads to data inconsistency.


• Difficulty of accessing data: Conventional file –processing environment do not allow needed data to be retrieved in a convenient and efficient way. More responsive data – retrieval systems are required for general use.


• Data isolation: Data are scattered in various files and files may be in different formats, writing new application programs to retrieve the appropriate data is difficult.

• Integrity problems: The data values stored in the database must satisfy certain types of consistency constrains. When new constrains are added, it is difficult to change the programs to enforce them. The problem is compounded when constraints involve several data items from different files.


• Atomicity problems: A computer system, like any other mechanical or electrical device, is subject to failure. In many applications, it is crucial that, if a failure occurs, the data be restores to the consistent state that existed prior to failure. Consider a program of fund transfer, it is essential to database consistency that either both the credit and debit occur or that neither occur. The fund transfer must be atomic – it must happen in its entirety or not at all. It is difficult to ensure atomicity in a conventional file – processing system.


• Concurrent access anomalies: The overall performance of the system and faster response many system allow multiple users to update the data simultaneously. In such an environment, interaction of concurrent updates is possible and may result in inconsistent data.


• Security problems: Not every user of the database system should be able to access all the data. As a result the data are secured in this system. But in the file – processing system security constraints is difficult.

Q. What do you mean by data abstraction? Describe the different levels of data abstraction.

Ans: Data abstraction: A major purpose of a database system is to provide users with an abstract view of the data. That is, the system hides certain details of how the data stored and maintained. There are three levels in data abstraction.

• Physical level: The lowest level of abstraction describes how the data are actually stored. The physical level describes complex low-level data structures in detail.

• Logical level: The next higher level of abstraction describes what data are stored in the database and what relationships exist among those data. The logical level thus describes the entire database in terms of a small number of relatively simple structures. The logical level of abstraction is used by database administrators, who must decide what information is to be kept in the database.


• View level: The highest level of abstraction describes only part of the entire database. Despite the use of simpler structures at the logical level, some complexity remains, because of the large size of the database. All users of the database system do not need all this information. They need to access only a part of the database. The view level of abstraction exists to simplify their interaction with the system. The system may provide many views for the same database.