DATABASE & SQL/PLSQL

Introduction (Database)
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DATABASE

Database is an important constituent for collection and storage of data. Data provided information after processing on them. To managing record and important data Databases server used. Database transaction is a unit of work in database management system. Database transaction must follow term ACID (Atomicity, Consistency, Isolation, Durability). Any transaction started then till it completion, that transaction follow ACID, for successful and no error.

The ability to modify the schema definition in one level should not affect the schema definition in the next higher level is called Data Independence. The application is independent of the storage structure and access strategy of data.

 

Two types of Data Independence are:

  1. Logical Data Independence: It is more difficult to achieve. Modification in logical level should affect the view level.
  2. Physical Data Independence: Modification in physical level should not affect the logical level.
ACID Property

Means atomicity, consistency, isolation and durability.

Atomicity: Each transaction is said to be “atomic". Means if any part of transaction fail then whole transaction failed. In a word say "ALL OR NOTHING". The failure of transaction commonly depend on Hard disk fail, System crash.

Consistency: Consistency means only valid data will be written to the database. If any transaction violate any consistency rule of database the whole transaction is Rollback, means it starts again from beginning. Each state in transaction must be consistent for database.

Isolation: If two transactions occurs at a time then both are not impact to each other, separate completion of each transaction. The isolation property does not ensure which transaction will execute first, merely that they will not interfere with each other. If second transaction depend on first then it will done, and data update after completion of traction.

Durability: Ensures that any transaction committed to the database will not be lost. Durability is ensured through the use of database backups and transaction logs that facilitate the restoration of committed transactions in spite of any subsequent software or hardware failures.

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