substract last and penultimate value

[Anonymous]

hi,

I have a table with the fields CodCliente, CodProd and Fecha.

I want to subtract the last value to the penultimate Fecha group by CodCliente and CodProd and next make the average of the substract.

This is the table:

CodCliente	CodProd	Fecha
1	1	30/04/2012
1	1	07/05/2012
1	1	15/05/2012
1	2	26/05/2012
1	2	18/06/2012
2	1	02/07/2012
2	1	25/08/2012
2	1	09/10/2012
2	2	15/10/2012
2	2	23/10/2012
2	2	02/11/2012
2	2	14/11/2012

I need substract for the CodCliente 1 and CodProd 1 15/05/2012 - 07/05/2012 and next  substract for the CodCliente 1 and CodProd 2 18/06/2012 - 26/05/2012 and calculate de average of this.

Regards

[Gysbert_Wassenaar]

See attached example.

[Anonymous]

Thanks,Gysbert, the example is exactly that i needed.

Regards