Can there be more than one database in an Oracle instance?
An Oracle instance can only have one database attached to it. Oracle forces this by placing locks on each of the database's datafiles so that no other instance can mount those datafiles, thus cannot mount the database.
The exception to this is if you are running Oracle Parallel Server, which has been renamed to Oracle Real Application Clusters for Oracle 9i. This lets you have multiple instances all connected to the database.
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