title: Error #1248 - Every derived table must have its own alias
body: <p>I'm getting the following error while running this query: Error #1248 - Every derived table must have its own alias </p>
  
  <pre><code>SELECT free_from, free_until
  FROM (
  SELECT a.end AS free_from,
  (SELECT MIN(c.start)
   FROM bookings c
   WHERE c.start&gt;a.end) as free_until
  FROM bookings a
  WHERE NOT EXISTS (
    SELECT 1
    FROM bookings b
    WHERE b.start BETWEEN a.end AND a.end + INTERVAL your_duration HOURS
  )
  AND a.end BETWEEN start_of_search_window AND end_of_search_window
  )
  ORDER BY free_until-free_from
  LIMIT 0,3;
  </code></pre>