mysql语句实现年龄分布

select nnd as '年龄段',count(*) as '人数' from(
 select 
 case
  when age>=1 and age<=10 then '1-10'
  when age>=11 and age<=20 then '11-20'
  when age>=21 and age<=30 then '21-30'
  when age>=31 and age<=40 then '31-40'
  when age>=41 and age<=50 then '41-50'
  when age>=51 and age<=60 then '51-60'
  when age>=61 and age<=70 then '61-70'
  when age>=71 and age<=80 then '71-80'
  when age>=81 and age<=90 then '81-90'
  when age>=91 and age<=100 then '91-100'
 ELSE '无年龄' end
 as nnd from doc_info
)a group by nnd