What is transaction in sql server ?? Why it is needed??

Transactions group a set of tasks into a single execution unit. Each transaction begins with a specific task and ends when all the tasks in the group successfully complete. If any of the tasks fails, the transaction fails. Therefore, a transaction has only two results: success or failure. Incomplete steps result in the failure of the transaction.

Users can group two or more Transact-SQL statements into a single transaction using the following statements:

• Begin Transaction
  
• Rollback Transaction
  
• Commit Transaction

If anything goes wrong with any of the grouped statements, all changes need to be aborted. The process of reversing changes is called rollback in SQL Server terminology. If everything is in order with all statements within a single transaction, all changes are recorded together in the database. In SQL Server terminology, we say that these changes are committed to the database.

Properties of Transactions:

Transactions have the following four standard properties, usually referred to by the acronym ACID:

• Atomicity: ensures that all operations within the work unit are completed successfully; otherwise, the transaction is aborted at the point of failure, and previous operations are rolled back to their former state.
  
• Consistency: ensures that the database properly changes states upon a successfully committed transaction.
  
• Isolation: enables transactions to operate independently of and transparent to each other.
  
• Durability: ensures that the result or effect of a committed transaction persists in case of a system failure.