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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.
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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.