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QUIESCE DATABASE results in all objects in the database being in the quiesced mode. Only the allowed user/group and sysadm, sysmaint,dbadm, or sysctrl will be able to access the database or its objects.

QUIESCE INSTANCE instance-name means the instance and the databases in the instance instance-name will be in quiesced mode. The instance will be accessible just for sysadm, sysmaint, and sysctrl and allowed user/group.