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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 update the meters.startTime to the lowest matching readings.readingTime in 1 sql query.

How do I do this?

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