title: mysql: select all items from table A if not exist in table B
body: <p>I am having problem with selecting values from table a (id, room_name) where there are no corresponding events in table b (room_id, room_start, room_finish)</p>
  
  <p>my query looks following </p>
  
  <pre>
  SELECT id, room_name FROM rooms 
  WHERE NOT EXISTS 
  (SELECT * FROM room_events 
      WHERE room_start BETWEEN '1294727400' AND '1294729200' 
            OR 
            room_finish BETWEEN '1294727400' AND '1294729200')
  </pre>
  
  <p>table a contains multiple rooms, table b contains room events
  I am getting no results in case there is any event for any of the rooms within the timestamps. I am expecting all rooms having NO events.</p>