SELECT marker.*,
SUM(IFNULL(marker.MAR_USER = map.MAP_USER,0)) AS owned
FROM `marker`
LEFT JOIN map_content ON (marker.MAR_ID=map_content.MAPC_MARKER)
LEFT JOIN map ON (map_content.MAPC_MAP=map.MAP_ID AND (map_id IS NULL OR map_id<>63))
WHERE marker.MAR_USER=18
GROUP BY marker.MAR_ID
HAVING owned=0