title: SQL: Reference one to one-of-many
body: <p>I'm having what some would call a rather strange problem/question.</p>
  
  <p>Suppose I have a table, which may reference one (and only one) of many different other tables. How would I do that in the <strong>best</strong> way?? I'm looking for a solution which should work in a majority of databases (MS SQL, MySQL, PostgreSQL etc). The way I see it, there are a couple of different solutions (is any better than the other?):</p>
  
  <ol>
  <li>Have one column for each possible reference. Only one of these columns may contain a value for any given row, all others are null. Allows for strict foreign keys, but it gets tedious when the number of "many" (possible referenced tables) gets large</li>
  <li>Have a two column relationship, i.e. one column "describing" which table is referenced, and one referencing the instance (row in that table). Easily extended when the number of "many" (referenced tables) grows, though I can't perform single query lookup in a straightforward way (either left join all possible tables, or union multiple queries which joins towards one table each)</li>
  <li>??</li>
  </ol>
  
  <p>Make sense? What's best practise (if any) in this case?
  I specifically want to be able to query data from the referenced entity, without really knowing which of the tables are being referenced.</p>
  
  <p>How would you do?</p>