21 - 即時食物配送 II

-- sum(if(order_date=customer_pref_delivery_date,1,0))/count(*)=sum(order_date=customer_pref_delivery_date)/count(*)
-- count(*),表示數據的行數，如果有分組，為分組后數據的行數select round(100*sum(if(order_date=customer_pref_delivery_date,1,0))/count(*),2) immediate_percentage 
fromDelivery 
where (customer_id,order_date) in (selectcustomer_id,min(order_date)from Deliverygroup bycustomer_id);