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A database in the context of Oracle is a collection of files (physical structures):

  • data files,
  • control files,
  • redo log files
  • …

A database may be mounted and opened by many instances.

This physical structures contain logical structures:

  • tablespaces,
  • schema objects,
  • data blocks,
  • extents,
  • and segments.

Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.

Data Dictionary

select   
   db_unique_name,
   database_role,
   open_mode
from      
   v$database

Status / Open

SQL> select status from v$instance;

STATUS
------------
MOUNTED

SQL> alter database open;

Database altered.

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