UPDATE distance_m dest,
(
  SELECT `distance_m`.*
  FROM `distance_m`
  WHERE `distance_m`.`elevator_id` = #{source_id}
  AND ( distance is not NULL and distance > 0)
) source
SET dest.distance = source.distance
WHERE dest.elevator_id = #{dest_id}
AND ( dest.distance is NULL or dest.distance <= 0)
AND dest.city_id = source.city_id")