select 等级,count(*) as 学生人数
from 
(select *,case when 分数<60 then '不及格'
when 分数<=70 then '及格' 
when 分数<=85 then '良好' 
when 分数<=100 then '优秀' 
end as 等级
from 学生分数表
) as new1
group by 等级;