title: Find free room (booking system)
body: <p>For a booking system I have a table with rooms, arrival and departure.</p>
  
  <p>Example data:</p>
  
  <pre><code>id | room | arrival    | departure
  ---+------+------------+-----------
  1  | 1    | 2011-03-12 | 2011-03-14
  2  | 1    | 2011-03-08 | 2011-03-09
  3  | 1    | 2011-03-19 | 2011-03-20
  4  | 2    | 2011-03-22 | 2011-03-30 
  5  | 2    | 2011-03-19 | 2011-03-22
  </code></pre>
  
  <p>My question is now: if I have a new booking (<code>2011-03-10</code> to <code>2011-03-12</code>), how can I check which rooms are free?</p>
  
  <p>Output for this example should be room <code>1</code> and <code>2</code>.   </p>