Variation of past and current sales

[v!ctor]

Hello
I have build an inventory control software that consists of
Inventory (in the sample attached file is called "Αρχείο Ειδών")
Purchases-Orders(Αγορές - Πωλήσεις)
Transactions (Είδος Συναλλαγής)
Past Sales (Πωλήσεις Προηγούμενου)

and a query having all the sales by month.

I am trying to find
a. how to get the variation of past sales and current sales of each month.
b. hot to get the variation of average of completed months (e.g now Jan. and Feb.) of past sales and current sales.

Enclosed pleased find a sample of my work



Hope to be understandable

[orange]

There seems to be something wrong with the zip according to Winzip.
You may find some info here
http://allenbrowne.com/subquery-01.html
http://allenbrowne.com/subquery-02.html

[v!ctor]

Sample.zip

I uploaded the file again