I have a table with one number field.

I have a second table with another number field, which relates to the first.

Say I enter the number 100 in the first table (which is the number of items received).

Then I create 2 different entries in the 2nd table, one with the number 10 and another one with 20 (It means that from the 100, I used the 10 items in entry 1 and 20 items in entry 2).

I need to make a calculation (and store it in table 1 if it is possible, as a different field) that gives me the remaining number of items from table 1. That means it subtracts from 100 the values 10 and 20 to give 70.

I hope I made it clear.

Thanks.

table_a::value - sum ( table_b::value )

~jens