> OK, here are a few variations (assuming Fb 1.5).
>
> invoice table
> 1. invid
> 2. customerid
> 3. invdate
> 4. totalamount
>
> 1. For a given customer and month:
>
> select avg(totalamount) as mthly_avg
> from invoice
> where customerid = :customerid
> and invdate between :startdate and :enddate;
>
> 2. For all customers in a given calendar month:
>
> select customerid, avg(totalamount) as mthy_avg
> from invoice
> where extract(month from invdate) = :aMonth
> group by customerid;
>
> 3. For all customers, by month, for a given period:
> select
> extract(year from invdate) as yearno,
> extract(month from invdate) as monthno,
> customerid,
> avg(totalamount) as mthy_avg
> where invdate between :startdate and :enddate
> group by 3, 1, 2
> order by 3, 1, 2
>
> /helen

Iam sorry Helen if I didn't make myself clear. I have try your
suggestion and it doesn't give me the result that I want. The sql
above result to the average totalamount for each customer per month.
What I need the the average monthly totalamount. Can this be
accomplish without the need to enter parameter? Below I a sql
statement that I have try to make and I feel a bit awful about it, if
it is wrong can you correct or improve it.

select "Cust ID", sum("Grand Total") / (cast (max("Deliver Date") -
min("Deliver Date") as numeric(15,2)) / 30)
from "DR"
group by "Cust ID"
having max("Deliver Date") - min("Deliver Date") != 0

* please me reminded that there are some customer that has no invoice
in a month which should be considered in this computation.

Thanks a lot Helen. This puzzle has been taking me so long to complete it.

Regards,

James