with t1 as(select date(visit_time) as date,count(distinct user_id) as visits
from visit_tb
group by date(visit_time) ),

t2 as(select date(order_time) as date,count(distinct user_id) as orders
from order_tb
group by date(order_time) )

select t1.date,concat(round((orders/visits)*100,1),'%') as cr
from t1
left join t2 on t1.date=t2.date
order by t1.date

我自己解答出来了难题！我太厉害了！