Make a query that reads the previous month's readings
(meter_ID and reading)and a second similar one that reads
the current month's.
Then make a third query, add the previous two and join
them on the meter_ID field. Select the meter_ID field for
output, the two readings if you want them, and add a
calculated field to subtract the two.

HTH,
Nikos

These queries are always based on horrendous SQL -- sometimes the nested-
querydef solution is the most practical even if not the fastest.

You probably need something like (not tested!!):-

 SELECT a.MeterNumber,
     a.DtRead,
     a.ReadValue - b.ReadValue AS Difference
 FROM Readings AS a, Readings AS b
 WHERE a.MeterNumber = b.MeterNumber
   AND b.DtRead = (  
       SELECT MAX(c.DtRead)
       FROM Readings AS c
       WHERE a.MeterNumber = c.MeterNumber
         AND a.DtRead > c.DtRead
       )
 SORT BY a.MeterNumber, a.DtRead

I think there is a way of sqeezing out one of the table references, but
it's the end of a long day here... :-)  There are loads of SQL gurus on
m.p.a.adpsqlserver

Hope that helps a bit

Tim F