Column as result of subtraction of two other columns?

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.

Many thanks,

Mark.

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Mark Cave-Ayland
Webbased Ltd.
Tamar Science Park
Derriford
Plymouth
PL6 8BX
England

Tel: +44 (0)1752 764445
Fax: +44 (0)1752 764446

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