Shutting down a d/b but not the instance

I need help in understanding the diff between shutting down a d/b and shutting down an instance.

When we startup an instance using startup nomount, we have an instance in the memory. Only when we perform alter database open will the database actually get mounted in that instance.

Now, how can I dismount/close the current d/b and open another one in the same instance w/o shutting down the instance.

I'm learning the basics of Oracle. So any help will be welcome.

You can't. The instance applies to a specific db. You can't switch the instance to reference another db while running.

However, you can have multiple instances affecting the same DB (Parallel Server), and multiple DB/instance pairs referencing synchronized copies of the same data.

So after starting an instance but before opening a d/b, I can select any of the d/b to be opened. Once I open a d/b then I have to shutdown the entire instance before I can open another d/b in the same instance.

Do I understand this correctly !!

I start the instance with the foll:
svrmgrl>startup nomount;

And I mount/open a d/b with the foll:
svrmgrl>alter database mount; or
svrmgrl>alter database open;

How can I select which d/b I want mounted in the instance both on NT and Solaris?

You can select which instance to mount by setting your ORACLE_SID environment to the appropriate instance you want and then run svrmgrl