1)Instance Started
2)Database Mounted
3)Database Opened
4)Database Started.
Pick up a book on Oracle Concepts/Architecture and Admin/Backup and Recovery for a more complete answer.
Startup Nomount : -
Read the parameter file and start the instance.
Instance name identified by ORACLE_SID env parameter.
The parameter file controls such things as SGA size, database name that can connect to this instance.
Database not associated with instance yet. <br/>
As an example, you may need to do this if you will mount a standby database.<br/><br/>
Mount : -
Associates database with the instance.
Open and read control files
Establish physical files associated with database. i.e location, size,
amount of redo log files. Physical database file names and locations.
DBA may need to be in mount state for all kinds of DBA activity.
examples :
rename/move system datafile.
put database in archivelog mode.
Database not yet available for user connections.
Opened/started
Ensure physical files exist.
make available for normal operations.
open online datafiles, redo logs.
acquire rollback segments.
These states are useful in many situations to a DBA, main areas where these are useful are
Architecture/Admin
Backup/Recovery.
