referencing column aliases in select list

Hi,

I have a SELECT statement that is a bit involved in terms of
calculations, so I wanted to set up some column aliases and refer to
them further down the select list:

---<---------------cut here---------------start-------------->---
SELECT table1.col1 - table2.col1 AS diff1,
table1.col2 + table2.col2 AS sum1,
sum1 - diff1
FROM table1 INNER JOIN table2 ON (table1.id = table2.id)
---<---------------cut here---------------end---------------->---

but this fails with the error message that sum1 column is not known.
How can one get around this? The calculations are quite involved and
would be difficult and error-prone to repeat them anywhere they're
needed in the select list. Thanks in advance for any pointers.

Cheers,

--
Seb