一个很简单的做法

select distinct date(order_time) dt, 
(select round(sum(if(start_time is null, 0, 1))/7, 2) from tb_get_car_order where timestampdiff(day, date(order_time), date(torder.order_time)) between 0 and 6) finish_num_7d, 
(select round(sum(if(start_time is null, 1, 0))/7, 2) from tb_get_car_order where timestampdiff(day, date(order_time), date(torder.order_time)) between 0 and 6) cancel_num_7d
from tb_get_car_order torder
where date(order_time) between '2021-10-01' and '2021-10-03'
order by 1