select b.* 
from (select job,
    floor((count(*)+1)/2) as start,
    floor(count(*)/2+1) as end
from grade  
group by job
order by job) a, (
select id, job, score, rank() over(partition by job order by score desc) as t_rank
from grade ) b
where a.job = b.job and (a.start = b.t_rank or a.end = b.t_rank)
order by id