What is the difference between startup Mount;startup Nomount;startup The three statements that start the database?

1, STARTUP Nomount
The Nonount option simply creates an Oracle instance, and when the instance is opened, the system displays a list of the SGA memory structure and size;
2. STARTUP MOUNT
This command creates an instance and installs the database, but does not open the database, in this way, in addition to seeing the SGA system list, the system will also give the "Database load complete" hint;
3, STARTUP
This command completes all three steps of creating an instance, installing an instance, and opening a database, in which case the system can see all the prompts in front of the startup Mount, and a "database is turned on" prompt, at which point the database system is in a normal working state and can accept user requests.

Rman> Startup Nomount
Connected to targetDatabase(Not started)
Oracle routines Started
System global Zone total 135338868 bytes
Fixed Size 453492 bytes
Variable Size 109051904 bytes
Database buffers 25165824 bytes
Redo buffers 667648 bytes rman> startup mount
Connected to target database (not started)
Oracle routines Started
The database is loaded
System global Zone total 135338868 bytes
Fixed Size 453492 bytes
Variable Size 109051904 bytes
Database buffers 25165824 bytes
Redo buffers 667648 bytes rman> Startup
Connected to target database (not started)
Oracle routines Started
The database is loaded
Database is open
System global Zone total 135338868 bytes
Fixed Size 453492 bytes
Variable Size 109051904 bytes
Database buffers 25165824 bytes
Redo buffers 667648 bytes above are three types of screenshots, startup Nomount just started the instance without starting the database, startup Mount started the instance and loaded the database, but the database did not open, s Tartup is the most complete, instance, database loaded, database open all completed. For example: Microsoft Windows XP [version 5.1.2600]
(C) All rights reserved 1985-2001 Microsoft Corp. C:/Documents and Settings/administrator>cd/c:/>sqlplus/nolog Sql*plus:release  9.2.0.1.0-production on Monday September 15:21:50 2009 Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved. Sql> connect/@dwj as Sysdba
is connected.
Sql> Startup Nomount
ORA-01081: Unable to start an ORACLE---that is already running please close first
sql> Shutdown Abort
The ORACLE routine has been closed.
Sql> Startup Nomount
The ORACLE routine has started. Total System Global area 135338868 bytes
Fixed Size 453492 bytes
Variable Size 109051904 bytes
Database buffers 25165824 bytes
Redo buffers 667648 bytes
Sql>