specific mysql update based on group by data

Question

This one has me stumpped.

I have 2 tables as so:

METERS
id | startTime

READINGS
id | meter_id | readingTime

What I want to do is u

Answer 1

Like this:

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;

Answer 2

UPDATE METERS m SET startTime = (SELECT MIN(r.readingTime)
    FROM READINGS r
    WHERE r.meter_id = m.id)
WHERE m.id = your_id