Custom field populated by sql call to non sugar database

I have a custom field that I would like to populate its value with a sql call to a NON sugar database.  does anybody have a code example?

An example might help.  1 Product has different agreed upon pricing for each customer.  So I know the product and the customer as entered in sugar.  I want to call out to the pricing table to find their specific price.  Obviously an easy database set of tables that one would query to get this info.  Once I get the price, I want that price stored in sugar database field.

I appreciate your help in advance