During a ROLLBACK, the database system undoes all changes that a transaction executed in the database. To do this, the database system uses the transaction's undo log entries.
The database system then releases all locks that have been allocated to the transaction, implicitly deletes all results tables that have been generated by the transaction, and opens a new transaction.
Any changes that are have been completed by a COMMIT can no longer be reversed by a ROLLBACK.
A ROLLBACK can be explicitly requested using the ROLLBACK statement.