How do write a query...

Hello,

I have the following problem :

I have a table like
Id Num Date AValue
1 10 01/01/2005 50
2 10 31/05/2005 60
3 25 02/02/2005 55
4 25 15/03/2005 43
5 25 28/05/2005 62
etc..

Id is unique, Num is an identification number with duplicates possible,
date is a ... date and Avalue... a value!

If we have
Id Num Date AValue
Id1 Num1 Date1 AValue1
Id2 Num1 Date2 AValue2

The table is ordered on Num+Date.
What I would like to calculate is (AValue2-AValue1) for a given Num
(here num1).

In this case, I would have to calculate
60-50 for Num 10
and
43-55, 62-43 for Num 25.

Do you have any idea if it can be done simply with a request...

I thank you

Regards.

Alain Reymond