21 - 即时食物配送 II

-- sum(if(order_date=customer_pref_delivery_date,1,0))/count(*)=sum(order_date=customer_pref_delivery_date)/count(*)
-- count(*),表示数据的行数，如果有分组，为分组后数据的行数


select 
    round(100*sum(if(order_date=customer_pref_delivery_date,1,0))/count(*),2) immediate_percentage 
from
    Delivery 
where 
    (customer_id,order_date) in 
        (select
            customer_id,min(order_date)
        from 
            Delivery
        group by
            customer_id);