Re: Column as result of subtraction of two other columns?

On Fri, Jul 16, 2004 at 15:31:33 +0100,
Mark Cave-Ayland <m(dot)cave-ayland(at)webbased(dot)co(dot)uk> wrote:
> Hi everyone,
>
> I'm trying to calculate an output column which is the difference of two
> other columns in the query output; the first column is an aggregate of
> items in stock, while the second column is an aggregate of items which
> have been used. The third column should should be the difference of the
> two values so I can then output all three columns in a table.
>
> Unfortunately I can't get this to work at the moment :(. I've simplified
> the query down to the following:
>
> dev=# select 1 as a, 2 as b, (b - a) as c;
> ERROR: column "b" does not exist
> dev=#
>
> Do I need to create some form of alias so the calculation can see the
> other columns? I am using PostgreSQL 7.4.2 on Linux.

You can't use column aliases in other columns; you need to repeat the
column expressions.