1174. Immediate Food Delivery II.sql

-- Question
-- +-----------------------------+---------+
-- | Column Name                 | Type    |
-- +-----------------------------+---------+
-- | delivery_id                 | int     |
-- | customer_id                 | int     |
-- | order_date                  | date    |
-- | customer_pref_delivery_date | date    |
-- +-----------------------------+---------+
-- 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:
-- +-------------+-------------+------------+-----------------------------+
-- | delivery_id | customer_id | order_date | customer_pref_delivery_date |
-- +-------------+-------------+------------+-----------------------------+
-- | 1           | 1           | 2019-08-01 | 2019-08-02                  |
-- | 2           | 2           | 2019-08-02 | 2019-08-02                  |
-- | 3           | 1           | 2019-08-11 | 2019-08-12                  |
-- | 4           | 3           | 2019-08-24 | 2019-08-24                  |
-- | 5           | 3           | 2019-08-21 | 2019-08-22                  |
-- | 6           | 2           | 2019-08-11 | 2019-08-13                  |
-- | 7           | 4           | 2019-08-09 | 2019-08-09                  |
-- +-------------+-------------+------------+-----------------------------+
-- Output: 
-- +----------------------+
-- | immediate_percentage |
-- +----------------------+
-- | 50.00                |
-- +----------------------+
-- 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.

-- Solution
select 
    round(count(case when fo.first_order = d.customer_pref_delivery_date then 1 end)/count(d.customer_id)*100, 2) immediate_percentage
from Delivery d
join (
    select 
    customer_id,
    min(order_date) first_order
    from Delivery
    group by customer_id
) fo
on d.order_date = fo.first_order
and d.customer_id = fo.customer_id;