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.
1. Reduces Data Redundancy
2. Maintains Data Consistent
3.Query to access data
5. Easier access by tables
1. Redundant Data
3. Not easy to access data.
4. No security
5. Concurrent access is not allowed.
Physical Level - This is the lowest level of data abstraction which describes which describes how the data is to be stored in the database.
Logical level - The level describes the data in the database as well as relations between these database.
View Level- This is the highest level of data abstraction , only some part of actual database is viewed to the user.
There are two Integrity rules.
> Entity Integrity: States that â€œPrimary key cannot have NULL valueâ€
> Referential Integrity: States that â€œForeign 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