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sql - specific mysql update based on group by data

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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by (71.8m points)

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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