title: How to check if a table is empty and if so, check in another table?
body: <p>I have two views: <code>current_campaign</code> and <code>last_campaign</code>:<br>
  <code>current_campaign</code> always has one row or none.<br>
  <code>last_campaign</code> always has one row.</p>
  
  <p>I have another view that needs to get information from one of these:  <code>contributors</code>. </p>
  
  <p>If <code>current_campaign</code> has one row, I need to give it the preference.<br>
  If <code>current_campaign</code> is empty, then I can get information from <code>last_campaign</code>.<br>
  Is that possible?</p>