title: ISNULL equivalent for empty fields
body: <p>Is there something like ISNULL() OR COALESCE() but not that checks for null value but for an empty value.</p>
  
  <p>for example: </p>
  
  <pre><code>  SELECT cu.last_name, cu.first_name, cu.email, hu.email FROM 
   (SELECT DISTINCT c.first_name, c.last_name, c.email, c.household_id, h.head_of_household_id
    FROM rd_customers c
    JOIN rd_households h ON c.household_id = h.household_id
    JOIN ad_registrations r ON r.customer_id = c.customer_id
    JOIN ad_meeting_times a ON  r.session_id = a.session_id and a.meeting_time_id = 203731) cu
    LEFT JOIN rd_customers hu ON hu.customer_id = cu.head_of_household_id
  </code></pre>
  
  <p>instead of returning four columns I'd like to get three and the third one suppose to have values either of cu.email or hu.email if the first one is EMPTY!
  ISNULL and COALESCE don't work here i dunno why</p>