FreeRadius overview

I have gotten freeRadius working with a mikrotik NAS using the users flat file (hooray for me! <g>). Now I need to have the freeRadius server reference a MS-SQL db server rather than the users file on the freeRadius server. I am quite fuzzy about the process tho. I am hoping for some basic edukashun. How do you tell freeRadius to reference an external MS-SQL db rather than the local users flat file? How do you issue the query to the db? (what file contains the connection string and query) How do the returning fields get mapped into RADIUS attributes? (what do you do when the db calls the field "uname" and RADIUS calls it "user-name") Thank you for your time, Dave Covert