Re: Calculating the difference between result columns

"Keith Worthington" <keithw(at)narrowpathinc(dot)com> writes:
> I'm stuck again. I have written two queries. The first calculates the total
> quantity of items that have been ordered. The second calculates the total
> quantity of items that have been received. Now I want to find the difference
> which will tell me what is still on order.

Something like

SELECT o.item_id, ordered - received FROM
(SELECT ...) o
(SELECT ...) r
WHERE o.item_id = r.item_id;

I'm not sure how efficient this mess is going to be :-(. If you intend
to do this a lot, you might want to think about a different database
structure that doesn't require quite so much joining to find out
anything...

regards, tom lane