单挑力扣（LeetCode）SQL题：1596. 每位顾客最经常订购的商品（难度：中等）

题目：1596. 每位顾客最经常订购的商品

（通过次数5,117 | 提交次数6,850，通过率74.70%）

表：Customers
+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| customer_id   | int     |
| name          | varchar |
+---------------+---------+
customer_id 是该表主键
该表包含所有顾客的信息

表：Orders
+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| order_id      | int     |
| order_date    | date    |
| customer_id   | int     |
| product_id    | int     |
+---------------+---------+
order_id 是该表主键
该表包含顾客 customer_id 的订单信息
没有顾客会在一天内订购相同的商品 多于一次

表：Products
+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| product_id    | int     |
| product_name  | varchar |
| price         | int     |
+---------------+---------+
product_id 是该表主键
该表包含了所有商品的信息

写一个 SQL 语句，找到每一个顾客最经常订购的商品。
结果表单应该有每一位至少下过一次单的顾客 customer_id,他最经常订购的商品的product_id和product_name。
返回结果 没有顺序要求。
查询结果格式如下例所示。

示例 1：
输入：
Customers表：
+-------------+-------+
| customer_id | name  |
+-------------+-------+
| 1           | Alice |
| 2           | Bob   |
| 3           | Tom   |
| 4           | Jerry |
| 5           | John  |
+-------------+-------+
Orders表：
+----------+------------+-------------+------------+
| order_id | order_date | customer_id | product_id |
+----------+------------+-------------+------------+
| 1        | 2020-07-31 | 1           | 1          |
| 2        | 2020-07-30 | 2           | 2          |
| 3        | 2020-08-29 | 3           | 3          |
| 4        | 2020-07-29 | 4           | 1          |
| 5        | 2020-06-10 | 1           | 2          |
| 6        | 2020-08-01 | 2           | 1          |
| 7        | 2020-08-01 | 3           | 3          |
| 8        | 2020-08-03 | 1           | 2          |
| 9        | 2020-08-07 | 2           | 3          |
| 10       | 2020-07-15 | 1           | 2          |
+----------+------------+-------------+------------+
Products表：
+------------+--------------+-------+
| product_id | product_name | price |
+------------+--------------+-------+
| 1          | keyboard     | 120   |
| 2          | mouse        | 80    |
| 3          | screen       | 600   |
| 4          | hard disk    | 450   |
+------------+--------------+-------+
输出：
+-------------+------------+--------------+
| customer_id | product_id | product_name |
+-------------+------------+--------------+
| 1           | 2          | mouse        |
| 2           | 1          | keyboard     |
| 2           | 2          | mouse        |
| 2           | 3          | screen       |
| 3           | 3          | screen       |
| 4           | 1          | keyboard     |
+-------------+------------+--------------+
解释：
Alice (customer 1) 三次订购鼠标, 一次订购键盘, 所以鼠标是 Alice 最经常订购的商品.
Bob (customer 2) 一次订购键盘, 一次订购鼠标, 一次订购显示器, 所以这些都是 Bob 最经常订购的商品.
Tom (customer 3) 只两次订购显示器, 所以显示器是 Tom 最经常订购的商品.
Jerry (customer 4) 只一次订购键盘, 所以键盘是 Jerry 最经常订购的商品.
John (customer 5) 没有订购过商品, 所以我们并没有把 John 包含在结果表中.

来源：力扣（LeetCode）
链接：https://leetcode.cn/problems/the-most-frequently-ordered-products-for-each-customer

#测试数据
Create table If Not Exists Customers (customer_id int, name varchar(10));
Create table If Not Exists Orders (order_id int, order_date date, customer_id int, product_id int);
Create table If Not Exists Products (product_id int, product_name varchar(20), price int);

insert into Customers (customer_id, name) values ('1', 'Alice');
insert into Customers (customer_id, name) values ('2', 'Bob');
insert into Customers (customer_id, name) values ('3', 'Tom');
insert into Customers (customer_id, name) values ('4', 'Jerry');
insert into Customers (customer_id, name) values ('5', 'John');

insert into Orders (order_id, order_date, customer_id, product_id) values ('1', '2020-07-31', '1', '1');
insert into Orders (order_id, order_date, customer_id, product_id) values ('2', '2020-7-30', '2', '2');
insert into Orders (order_id, order_date, customer_id, product_id) values ('3', '2020-08-29', '3', '3');
insert into Orders (order_id, order_date, customer_id, product_id) values ('4', '2020-07-29', '4', '1');
insert into Orders (order_id, order_date, customer_id, product_id) values ('5', '2020-06-10', '1', '2');
insert into Orders (order_id, order_date, customer_id, product_id) values ('6', '2020-08-01', '2', '1');
insert into Orders (order_id, order_date, customer_id, product_id) values ('7', '2020-08-01', '3', '3');
insert into Orders (order_id, order_date, customer_id, product_id) values ('8', '2020-08-03', '1', '2');
insert into Orders (order_id, order_date, customer_id, product_id) values ('9', '2020-08-07', '2', '3');
insert into Orders (order_id, order_date, customer_id, product_id) values ('10', '2020-07-15', '1', '2');

insert into Products (product_id, product_name, price) values ('1', 'keyboard', '120');
insert into Products (product_id, product_name, price) values ('2', 'mouse', '80');
insert into Products (product_id, product_name, price) values ('3', 'screen', '600');
insert into Products (product_id, product_name, price) values ('4', 'hard disk', '450');

解题思路：

虽然题目看着很长，很唬人，但明显又是一道典型的分组排序题目，只不过排序的值需要预先计算出来。

根据题目要求，计算出每个顾客最经常购买的商品（购买次数最多）。

那么，第一步，我们需要计算出，每个顾客每个商品的购买次数。这很简单，直接对订单表（Orders）按顾客+商品分组汇总即可。

第二步，我们需要将第一步的结果，按顾客分组，然后以商品的购买次数进行排序（如果购买次数相同，则排序序号相同），取出序号为1的结果即可。

最后，再根据product_id关联出product_name即可。

参考SQL：

select
    b.customer_id,
    b.product_id,
    c.product_name
from (
    select
        customer_id,
        product_id,
        rank() over(partition by customer_id order by cn desc) rk
    from (
        select
            customer_id,product_id,count(1) cn
        from Orders
        group by customer_id,product_id
    )a
)b
left join Products c
on b.product_id = c.product_id
where rk = 1;