SELECT
HOUR(a.reading_on) As hr, max(a.id),a.reading_on
FROM
InverterReadings a
LEFT JOIN
InverterReadings b
ON
YEAR(a.reading_on)=YEAR(b.reading_on)
AND MONTH(a.reading_on)=MONTH(b.reading_on)
AND day(a.reading_on)=day(b.reading_on)
AND hour(a.reading_on)=hour(b.reading_on)
AND
a.reading_on < b.reading_on
WHERE
b.reading_on is null
group by a.reading_on;
Ukázka:http://sqlfiddle.com/#!2/49a69/14