LC 1045买下所有产品的客户Customers Who Bought All Products　中等

Customer 表记录客户购买的产品，Product 表是全部产品清单；找出买齐了所有产品的客户 id。

思路 按客户 COUNT(DISTINCT 产品)，HAVING 与产品总数比较。

SELECT C.customer_id
FROM Customer C
JOIN Product P ON C.product_key = P.product_key
GROUP BY C.customer_id
HAVING COUNT(DISTINCT C.product_key)
       = (SELECT COUNT(*) FROM Product);

数据表：Customer,Product　·　关键字：找出买齐了所有产品的客户 id