select ifnull(driver_id,'总体'), round(sum(grade)/count(driver_id),1)
from
(select driver_id 
from tb_get_car_order 
where start_time is null and 
finish_time is not null and 
date_format(order_time,'%Y%m')='202110') as tb1
left join tb_get_car_order
using(driver_id)
where grade is not null
group by driver_id
with rollup