select 订单数区间,count(*) as 人数
from 
(select *,case when 订单数<=2 then '0-2'
when 订单数<=5 then '3-5'
else'5单以上'
end as 订单数区间
from 订单表) as new1
group by 订单数区间
order by 订单数区间 asc;