title: Abstract/super/sub class-like structure for database design
body: <p>I've run my head against the wall a couple of times on this one. So I'm hoping for a little help in the right direction.</p>
  
  <p>I have a table with ORDERS, one with TRAINS, one with FLIGHTS and one with BUSES. Each order should have a single way of transportation. My design has up until now consisted of a field in the ORDERS table stating the type of transport (train, flight, bus) and a field containing the forreign key to the stated type of transport.</p>
  
  <p>Is there any better way to do this?</p>