#方法一：失败：顺序不对，开窗函数排序对结果产生了影响
#select first_name
#from
#(
#select first_name,
#row_number() over(order by first_name) num
#from employees
#) a
#where mod(num,2)=1
方法一：成功
select a.first_name
from 
employees a inner join
(
 select first_name,
 row_number() over(order by first_name) num
 from employees
) b
on a.first_name=b.first_name
where mod(b.num,2)=1