title: "polymorphism" for FOREIGN KEY constraints
body: <p>There is this field in a table:</p>
  
  <pre><code>room_id INT NOT NULL CONSTRAINT room_id_ref_room REFERENCES room
  </code></pre>
  
  <p>I have three 2 tables for two kinds of rooms: <code>standard_room</code> and <code>family_room</code></p>
  
  <p>How to do something like this:</p>
  
  <pre><code>room_id INT NOT NULL CONSTRAINT room_id_ref_room REFERENCES standard_room or family_room
  </code></pre>
  
  <p>I mean, <code>room_id</code> should reference <em>either</em> <code>standard_room</code> <em>or</em> <code>family_room</code>.<br>
  Is it possible to do so?</p>