1174. Immediate Food Delivery II

delivery_id is the column of unique values of this table.
The table holds information about food delivery to customers that make orders at some date and specify a preferred delivery date (on the same order date or after it).

If the customer's preferred delivery date is the same as the order date, then the order is called immediate; otherwise, it is called scheduled.

The first order of a customer is the order with the earliest order date that the customer made. It is guaranteed that a customer has precisely one first order.

Write a solution to find the percentage of immediate orders in the first orders of all customers, rounded to 2 decimal places.

The result format is in the following example.

Example 1:

Input:
Delivery table:

Output:

Explanation:
The customer id 1 has a first order with delivery id 1 and it is scheduled.
The customer id 2 has a first order with delivery id 2 and it is immediate.
The customer id 3 has a first order with delivery id 5 and it is scheduled.
The customer id 4 has a first order with delivery id 7 and it is immediate.
Hence, half the customers have immediate first orders.

SELECT ROUND(AVG(order_date = customer_pref_delivery_date)*100,2) AS immediate_percentage
FROM Delivery
WHERE (customer_id , order_date ) IN 
    (
        SELECT customer_id , min(order_date) AS order_date 
        FROM Delivery
        GROUP BY customer_id
    )

找到 customer_id ,最早的order_date
判斷他是scheduled 還是 immediate
把immediate加起來/全部筆數

學習點 : WHERE (customer_id , order_date ) IN 是指主表的customer_id , order_date 必須跟 SELECT customer_id , min(order_date) AS order_date FROM Delivery GROUP BY customer_id 副表相同