In english:
Retrieve current month's usage (using current date/month),
retrieve previous month's usage (using previous date/month),
subtract current - previous to get difference.
table is water_records
id, customer_id, water_records_date, water_records_usage
1, '1', '2010-02-01', '226600'
2, '1', '2010-01-01', '223000'
3, '4', '2010-01-05', '5600'
9, '4', '2010-02-19', '88000'
15, '5', '2010-02-15', '10060'
19, '5', '2009-12-02', '9000'
21, '9', '2010-02-19', '10500'
22, '9', '2009-12-02', '8900'
SELECT t1.water_records_usage
FROM water_records t1
INNER JOIN water_records t2
on t1.ID = t2.ID
WHERE month(t1.water_records_date) = month(DATE_ADD(now(), INTERVAL -1 month))
AND
WHERE month(t2.water_records_date) = month(DATE_ADD(now(), INTERVAL -2 month))
Which is not working.....