基于分组数据的特定mysql更新

这件事把我难住了

我有两张桌子：

METERS
id | startTime

READINGS
id | meter_id | readingTime

我要做的是在1 sql查询中将

仪表.startTime

更新为最低匹配

读数.readingTime

我该怎么做？

像这样：

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;

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