One instance and two databases?

Question

Hello, I've installed oracle database on my server. I've read a lot of documentation to learn the basic concepts.
Ok, I have a question. I have read that one instance only can manage ONE database. Is it true? If it's true, how on earth I can create two database? for example, I want to create database-one and database-two.
I would create these with CREATE DATABASE command. How does Instance know if I want to access to database-one or database-two. How I indicate it in initilitation parameter file of my instance?
In sumary, Can one instance works with two databases?

Answer 1

Look at the following output. If I do not set my ORACLE_SID variable, the 9.2.0.7 client connects to the 9.2.0.7 database instance. If I set my ORACLE_SID , the same 9.2.0.7 client now connects to the 10.2.0.1 database (the database specified in the ORACLE_SID variable).
C:\>set ORACLE_SID=
C:\>sqlplus kkishore/kkishore
SQL*Plus: Release 9.2.0.7.0 - Production on Mon Jul 17 09:27:55 2006
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.7.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.7.0 - Production
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.7.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.7.0 - Production
C:\>set ORACLE_SID=ORA10GR2
C:\>sqlplus kkishore/kkishore
SQL*Plus: Release 9.2.0.7.0 - Production on Mon Jul 17 09:29:31 2006
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL>Message was edited by:
Kamal Kishore