1173-ImmediateFoodDeliveryI.sql

-- 1173. Immediate Food Delivery I
-- Table: Delivery
-- +-----------------------------+---------+
-- | Column Name                 | Type    |
-- +-----------------------------+---------+
-- | delivery_id                 | int     |
-- | customer_id                 | int     |
-- | order_date                  | date    |
-- | customer_pref_delivery_date | date    |
-- +-----------------------------+---------+
-- delivery_id is the primary key (column with 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.
-- Write a solution to find the percentage of immediate orders in the table, 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           | 5           | 2019-08-02 | 2019-08-02                  |
-- | 3           | 1           | 2019-08-11 | 2019-08-11                  |
-- | 4           | 3           | 2019-08-24 | 2019-08-26                  |
-- | 5           | 4           | 2019-08-21 | 2019-08-22                  |
-- | 6           | 2           | 2019-08-11 | 2019-08-13                  |
-- +-------------+-------------+------------+-----------------------------+
-- Output: 
-- +----------------------+
-- | immediate_percentage |
-- +----------------------+
-- | 33.33                |
-- +----------------------+
-- Explanation: The orders with delivery id 2 and 3 are immediate while the others are scheduled.

-- Create table If Not Exists Delivery (delivery_id int, customer_id int, order_date date, customer_pref_delivery_date date)
-- Truncate table Delivery
-- insert into Delivery (delivery_id, customer_id, order_date, customer_pref_delivery_date) values ('1', '1', '2019-08-01', '2019-08-02')
-- insert into Delivery (delivery_id, customer_id, order_date, customer_pref_delivery_date) values ('2', '5', '2019-08-02', '2019-08-02')
-- insert into Delivery (delivery_id, customer_id, order_date, customer_pref_delivery_date) values ('3', '1', '2019-08-11', '2019-08-11')
-- insert into Delivery (delivery_id, customer_id, order_date, customer_pref_delivery_date) values ('4', '3', '2019-08-24', '2019-08-26')
-- insert into Delivery (delivery_id, customer_id, order_date, customer_pref_delivery_date) values ('5', '4', '2019-08-21', '2019-08-22')
-- insert into Delivery (delivery_id, customer_id, order_date, customer_pref_delivery_date) values ('6', '2', '2019-08-11', '2019-08-13')

-- Write your MySQL query statement below
SELECT
    ROUND(
        (-- 即时订单数
            SELECT 
                COUNT(*)
            FROM 
                Delivery
            WHERE 
                order_date = customer_pref_delivery_date 
        ) 
        / 
        ( -- 所有订单数
            SELECT 
                COUNT(*)
            FROM 
                Delivery
        ) * 100,
        2
    ) AS immediate_percentage 

-- best solution
SELECT 
    ROUND(
        SUM( -- 汇总即时订单数
            CASE 
                -- 如果顾客期望的配送日期和下单日期相同，则该订单称为 「即时订单」
                -- 如果是即时订单为 1 否则 为 0
                WHEN order_date = customer_pref_delivery_date THEN 1
                ELSE 0
            END
        )
        /
        COUNT(*)*100, 
        2
    ) AS immediate_percentage 
FROM Delivery;