Confusion over username/password

I'm trying to learn Oracle v7.3.3, and the supplied documentation doesn't always help. I'm particularly confused by several examples that I've seen that specify a logon username and password. In several places the documentation has examples that show a connection to the database being done with a command line specification of:

  username/password

In actually using the software, however, I always have to use:

  username/password_at_dbalias

where dbalias is a database alias as defined in TNSNAMES.ORA. This brings to mind several questions:

1. Why don't the examples in the Oracle documentation include a database alias?
2. How does the software know which database you want to connect to?
3. Is the username/password form (with no dbalias) only for use on the server system, and only when the ORACLE_SID env. variable is set to a particular instance?
4. Is it safe to sssume that anywhere in the documenation where Oracle specifies username/password I can safely use username/password_at_dbalias?

TIA, Gregg Sporar
i2 Technologies Received on Tue Nov 25 1997 - 00:00:00 CST