#思路：先将等级表进行分组，再跟回答表连接，最后做聚合计算即可。
select level_cut, count(level_cut) num
from
answer_tb join 
(select author_id, author_level, 
case when author_level between 1 and 2 then '1-2级'
when author_level between 3 and 4 then '3-4级'
else '5-6级' end as level_cut
from author_tb) as tb1
using(author_id)
where char_len>=100
group by level_cut
order by num desc