Automatically Fill up the field in Query/Form

[David]

I have a months record from that contains
Date Current Month Sales Previous Month Sales
1/1/03 1000
2/1/02 1500 1000
3/1/03 900 1500
I hope that list clarifies every thing that each month
when I fill up the form query if show Current Month Sales
field empty for the new entry and Previous Month Sales
field automatically copies previous months current
record. how do i solve this Previous Month Sales field
automatically.
I am new in quries designing please help me out.
Thanks.