需要注意不改变原表顺序，所以用原表连接现在的表
select
    e.first_name
from
employees e
join
(
    select
        first_name
        ,row_number() over (order by first_name) as rk
    from 
        employees
) a
on e.first_name = a.first_name
where a.rk%2=1