Counter change

I have a table with RowId(autonumber),ReportMonth(date), SerialNumber(text), and Counter(number)



I'm trying to create a query that will eventually be a Make table query.



I want to show for each ReportMonth and each SerialNumber, the increase or decrease of the Counter.



Issue is that there might not be a SerialNumber and Counter every ReportMonth. Sometimes they may skip a month or a brand new SerialNumber may be entered so that record should show the current Counter.



I'm looking for ideas how to accomplish this,

Scott

Counter change