思路：通过与原表连接达到不需要排序(感觉完全没必要)
select
a.first_name
from
employees e
left join 
(
    select
    first_name
    ,row_number()over(order by first_name) rk 
    from employees
) a
on e.first_name = a.first_name
where rk mod 2 = 1