I am able to find one prob.. the Value column is already a aggregated column.. Then don we need to take the maximum value instead of summing it up??
And also Lag function is not working in mine. so i think my database is a previous version.. i am also not sure about what version it is..
Sorry I don't understand your last question (or I didn't understand what exactly is stored in the table and what exactly you need).
In your example output you show only 1 row for month = 01 but there are three rows for that month available. So which difference would you like to see? The one between the first and the second value, or the second and the third, or the first and the third?