Recover Online Redo Log File Scenarios

1) Redo log is not CURRENT and archived
SQL> STARTUP MOUNT;
SQL> ALTER DATABASE CLEAR LOGFILE GROUP <group#>;
2) Redo log is not CURRENT and not archived
 -- Restore the DB from previous backup and open the DB in mount state.
SQL> RECOVER DATABASE USING BACKUP CONTROLFILE UNTIL CANCEL;
-- Apply all the archives available
SQL> ALTER DATABASE OPEN RESETLOGS;
In a similar scenario an RMAN restore will be as below:
RMAN> RESTORE CONTROLFILE FROM '<backup location>';
RMAN> ALTER DATABASE MOUNT;
RMAN> RESTORE DATABASE;
RMAN> RECOVER DATABASE UNTIL TIME "to_date('JUN 05 2017 19:00:00','MON DD YYYY HH24:MI:SS')";
RMAN> ALTER DATABASE OPEN RESETLOGS;
3) Redo log is CURRENT (DB was shut down cleanly)
If the CURRENT redo log is lost and if the DB is closed consistently, OPEN RESETLOGS can be issued directly without any transaction loss. It is advisable to take a full backup of DB immediately after the STARTUP.
4) Redo log is CURRENT (DB was not shut down cleanly)
RMAN> RESTORE CONTROLFILE FROM '<location>';
RMAN> ALTER DATABASE MOUNT;
RMAN> RESTORE DATABASE;
RMAN> RECOVER DATABASE UNTIL TIME "to_date('JUN 05 2017 19:00:00','MON DD YYYY HH24:MI:SS')";
RMAN> ALTER DATABASE OPEN RESETLOGS;