title: why we have to backup archive log after backup full DB
body: <p>i am learning about backup recovery on oracle. i see the script to backup for both DB and archive log. but i think backup archive log maybe can not use when recovering database. all data change in archive log, which was already backup, it in the backup up DB also.
  for exmaple:
  i backup DB at 1h00, finish at 2h00
  at 2h00, i backup archive log. finish at 2h10</p>
  <p>at 4h,DB corrupted, i have to recover DB from DB backup. i think archive log when i backup at 2h00 is not use in that case.</p>
  <p>if i incorrect, please correct me. what perpose when we do backup archive log.</p>