Mysql题目查询购买了全部商品的id

1、有两张表customer 和 product,查询有 product 表中两个商品的 customer_id

SELECT *
FROM  (SELECT DISTINCT customer_id FROM customer)a
WHERE NOT EXISTS(
  SELECT product_key
  FROM product b
  WHERE NOT EXISTS(
     SELECT *
     FROM customer c
     WHERE b.product_key = c.product_key AND a.customer_id=c.customer_id)
)