SQL subquery? sub-something?

I come to the SQL well again after not knowing exactly what I'm looking for
term-wise.

In order to list customers by regional sales manager and have necessary
relevant information, I have an SQL that joins four files together, the
customer master being the primary table.  It works well.  Fast as ... heck.

Anyway, this SQL result set will be the basis for further information
gathering, i.e., sales history, forecasts, etc, for several applications.  I
would like this "base" SQL to be "invisible", like I envision DDS join
logical files where the join logic is defined in the file definition; when I
read the file, I don't worry about it being the same join logic in different
programs that use it.  Whereas, with SQL, if I add the Sales History to the
SQL statement, I seem to think there's a risk of introducing a relationship
that changes the join logic.

I don't know if this makes sense.  Don't know if I'm looking at creating an
SQL view, then using that in another SQL statement.  Or subquery.  Or just
adding to the existing "base" SQL.  I would just like to have an idea of the
best way to design it, so that it's flexible for future use.

tia,
db