ROLLBACK 

In a ROLLBACK, all the changes made by a transaction or a subtransaction on the database instance are reversed.

·        Changes closed with a COMMIT can no longer be reversed with a ROLLBACK.

·        As a result of a ROLLBACK, a new transaction is implicitly opened.

In normal database operation, the database system performs the required ROLLBACK actions independently. However, ROLLBACK can also be explicitly requested using appropriate SQL statements.

In a restart, the system checks which transactions were canceled or closed with a ROLLBACK. The actions are these transactions are undone.