LeetCode MySQL 1045. 买下所有产品的客户

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

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

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 。

# Write your MySQL query statement below
select c.customer_id
from Customer c
group by c.customer_id
having (select distinct count(*) from Product) = count(distinct c.product_key)