Sum(quantity) when province is null

[sersal10]

Hi all,

I have 2 fields province and quantity, and need to find how much is the total quantity of the null provinces.

I am trying something like this but doesn´t work:

Sum({<province = {NULL}>} quantity)

or

Sum({<province={"=Len(province) = 0"}>} quantity)

Kind regards,

Sergio.

[sersal10]

I tried this and finally works. I think there is a much simpler way but it works 

Sum({<quantity ={"=Len(province) = 0"}>} quantity) - Sum({<province = {*} >} total quantity)

Many thanks

[Clement15]

Good if it works