with t as (select 客户id,
case
when count(distinct 运单号) between 0 and 5 then '0-5'
when count(distinct 运单号) between 6 and 10 then '6-10'
when count(distinct 运单号) between 11 and 20 then '11-20'
else '20以上'
end as 单量 
from 快递揽收表
where date_format(创建日期,'%Y-%m') = '2020-05'
group by 客户id)

select 单量,count(单量) as 客户数 from t
group by 单量
order by 单量