select first_name
from employees
where first_name in (select first_name --判断first_name是否在符合题目条件的表中
from(select first_name,row_number() over(order by first_name) as rank_number from employees)as a --创建窗口给排序后的行标数
where a.rank_number%2=1) --对标数判断为奇数，保留