refer to computed columns

Hi

Think of the following query:

1 select
2 country,
3 (select sum(salary) from employees) as totalSalary,
4 sum(salary) as countrySalary,
5 countrySalary / totalSalary as countryPct
6 from employees
7 group by country
8 order by country
;

I know it does not work, but is there a way to refer to
the "computed" columns in line 5, so that I do not have
to repeat the calculations again.

BTW, are the calculation done more then once if I specify

1 select
2 country,
3 (select sum(salary) from employees) as totalSalary,
4 sum(salary) as countrySalary,
5 sum(salary) / (select sum(salary) from employees) as countryPct
6 from employees
7 group by country
8 order by country
;

Thanks for the help

Bruno