Technical Interview Questions and Answers :: DBMS
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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.
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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.
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The database and DBMS software together are called as Database system.
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1. Reduces Data Redundancy
2. Maintains Data Consistent
3.Query to access data
4. Security
5. Easier access by tables
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1. Redundant Data
2. Inconsistency
3. Not easy to access data.
4. No security
5. Concurrent access is not allowed.
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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.
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There are two Integrity rules.
> Entity Integrity: States that “Primary key cannot have NULL valueâ€
> Referential Integrity: States that “Foreign Key can be either a NULL value or should be Primary Key value of other relation.
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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
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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.
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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