[Help-gnu-radius] question about radius

[耀逸]

Dear all:
Sorry for asking about these simple question..
I know the RADIUS can support different authentication methods
and obtain the user's authentication data from different places, like
system database and internal RADIUS database.
My question is
1. if I want to specify one place that the RADIUS can obtain the user's
data, what should I do?
2. If I want to specify the RADIUS to use a internal RADIUS database, where
is the internal RADIUS database's password file? is this way just use a file
that contains many "username password" or this is a true database?
3. IF I want to use SQL authentication, I must to specify the place that the
RADIUS can obtain user's data and specify where is the SQL server.
What should I do?

Please help me, thank you for your help