Experimente com funções analíticas . Algo assim deve fazer o truque:
SELECT meterid, dateread,
reading - LAG(reading, 1, 0) OVER(PARTITION BY meterid ORDER BY dateread)
FROM meter_reading
SELECT meterid, dateread,
reading - LAG(reading, 1, 0) OVER(PARTITION BY meterid ORDER BY dateread)
FROM meter_reading
