select level_cut,count(1) num
from(
    select 
    case
        when author_level >= 5 then "5-6级"
        when author_level >= 3 then "3-4级"
        else "1-2级"
    end level_cut
    from author_tb left join answer_tb using(author_id)
    where char_len >= 100
)t
group by level_cut
order by num desc