title: Multiple yet mutually exclusive foreign keys - is this the way to go?
body: <p>I have three tables: Users, Companies and Websites. 
  Users and companies have websites, and thus each user record has a foreign key into the Websites table. Also, each company record has a foreign key into the Websites table.</p>
  
  <p>Now I want to include foreign keys in the Websites table back into their respective "parent" records. How do I do that? Should I have two foreign keys in each website record, with one of them always NULL? Or is there another way to go?</p>