1045.(Medium)买下所有产品的客户

Customer 表：

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| customer_id | int     |
| product_key | int     |
+-------------+---------+
product_key 是 Customer 表的外键。

Product 表：

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| product_key | int     |
+-------------+---------+
product_key 是这张表的主键。

写一条 SQL 查询语句，从 Customer 表中查询购买了 Product 表中所有产品的客户的 id。

示例：

Customer 表：
+-------------+-------------+
| customer_id | product_key |
+-------------+-------------+
| 1           | 5           |
| 2           | 6           |
| 3           | 5           |
| 3           | 6           |
| 1           | 6           |
+-------------+-------------+

Product 表：
+-------------+
| product_key |
+-------------+
| 5           |
| 6           |
+-------------+

Result 表：
+-------------+
| customer_id |
+-------------+
| 1           |
| 3           |
+-------------+
购买了所有产品（5 和 6）的客户的 id 是 1 和 3 。

来源：力扣（LeetCode）

链接：https://leetcode-cn.com/problems/customers-who-bought-all-products

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Solution

本题强调了primary key 和foreign key，故直接可通过顾客「购买的商品种类数」 = 「商品种类总数」筛选。

select customer_id
from customer
group by 1
having count(distinct product_key) = (select count(distinct product_key) from Product)

Table Schema

Create table If Not Exists Customer (customer_id int, product_key int);
Create table Product (product_key int);
Truncate table Customer;
insert into Customer (customer_id, product_key) values ('1', '5');
insert into Customer (customer_id, product_key) values ('2', '6');
insert into Customer (customer_id, product_key) values ('3', '5');
insert into Customer (customer_id, product_key) values ('3', '6');
insert into Customer (customer_id, product_key) values ('1', '6');
Truncate table Product;
insert into Product (product_key) values ('5');
insert into Product (product_key) values ('6');