Re: Calculating the difference between result columns

"Keith Worthington" <keithw@narrowpathinc.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