select
first_name 
from
(    
    select
    first_name
    ,row_number()over(order by first_name ) r
    from employees
) a
where r%2!=0
###这个不会按照原表排序


select
e.first_name 
from employees e join
(    
    select
    first_name
    ,row_number()over(order by first_name ) r
    from employees
) a
on e.first_name=a.first_name
where r%2!=0
#这个是按原表排序，这个是答案