select
    date(ot.order_time) as date,
    concat(round(count(distinct ot.user_id)*100/count(distinct vt.user_id),1),'%') as cr
from 
    visit_tb vt left join order_tb ot
    on date(vt.visit_time) = date(ot.order_time)
group by date
order by date asc
;