Så här:
UPDATE Meters m
INNER JOIN
(
SELECT meter_id, MIN(reading_time) lowesttime
FROM readings
GROUP BY meter_id
) r ON m.id = r.meter_id
SET m.starttime = r.lowesttime;
Så här:
UPDATE Meters m
INNER JOIN
(
SELECT meter_id, MIN(reading_time) lowesttime
FROM readings
GROUP BY meter_id
) r ON m.id = r.meter_id
SET m.starttime = r.lowesttime;