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

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