In Microsoft SQL cross apply can be used for this in even more situations and with less repetition:

    select top(20)
        title,
        country,
        ...
        avg(gross_salary) as average_gross_salary,
        ...
    from employees
    cross apply ( select
        gross_salary = employees.salary + employees.payroll_tax, -- or "as .."
        gross_cost = ...
    ) v -- some name required but don't need to use it if column names are unique
    where ...