Well the short answer is sql directly no, buuuut :)
What you can do is use a radius server that supports the sql flavour you like, as a backend, i personally default to freeradius since it can be connected to most flavours of database either natively or via odbc, plus it's free and very customizable.
Here is a link that explains it quick and dirty
http://www.easysoft.com/applications/freeradius/odbc-authorization.html
The trick here is that if the default fields that the radius server looks for does not fit the customer tables, then you can just rewrite the sql sentences to fit your needs.
if you use something like an ACS that also has the possibility to work with external databases, though customizability is limited.
This should give you an idea on how to attack it :)
- Christian