来源：Leetcode  地址：https://leetcode-cn.com/problems/customers-who-bought-all-products/  【侵删】

ExampleA （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 。

解题思路（group_concat 子查询）

select n.customer_id from
(select customer_id,group_concat(distinct product_key order by product_key) as product_key from customer group by customer_id order by customer_id) as n where n.product_key<=>(select group_concat(product_key order by product_key) as product_key from Product);