select t1.date,
concat(round(ifnull(order_cnt,0)/visit_cnt * 100,1),'%') as cr
from(
select date_format(visit_time,'%Y-%m-%d') as date,
count(distinct user_id) as visit_cnt
from visit_tb
group by date_format(visit_time,'%Y-%m-%d')
) t1 
left join 
(
select date_format(order_time,'%Y-%m-%d') as date,
count(distinct user_id) as order_cnt
from order_tb
group by date_format(order_time,'%Y-%m-%d')
) t2 using(date)
order by date