Technical Interview Questions and Answers :: DBMS
A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose.
It is a collection of programs that enables the user to create and maintain a database. In other words, it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications.
The database and DBMS software together are called as Database system.
Advantage of DBMS are :
> Redundancy is controlled.
> Unauthorized access is restricted.
> Providing multiple user interfaces.
> Enforcing integrity constraints.
> Providing backup and recovery.
> Data redundancy & inconsistency.
> Difficult in accessing data.
> Data isolation.
> Data integrity.
> Concurrent access is not possible.
> Security Problems.
1. Physical level: The lowest level of abstraction describes how data are stored.
2. Logical level: The next higher level of abstraction, describes what data are stored in the database and what relationship among those data.
3. View level: The highest level of abstraction describes only part of entire database.
There are two Integrity rules.
> Entity Integrity: States that “Primary key cannot have NULL value”
> Referential Integrity: States that “Foreign Key can be either a NULL value or should be Primary Key value of other relation.
The description of data base is called in tension or database schema,which is specified during database design and is not expected to change frequently.
The actual data in a database may change quite frequently.The data in a database at a particular moment in time is called a extension or database state or snapshot.
Source:- Extension and intension
System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with the performance at least comparable to that of the existing system.
Its two subsystems are
> Research Storage
> System Relational Data System.
Unlike Relational systems in System R
> Domains are not supported
> Enforcement of candidate key uniqueness is optional
> Enforcement of entity integrity is optional
> Referential integrity is not enforced