[코드카타] SQL 44 Customers Who Bought All Products

[코드카타] SQL 44 Customers Who Bought All Products

44. Customers Who Bought All Products
https://leetcode.com/problems/customers-who-bought-all-products/

Write a solution to report the customer ids from the Customer 
table that bought all the products in the Product table.
Return the result table in any order.

Select customer_id
From Customer
group by customer_id
having count(distinct product_key) = (select count(product_key) from product)

product table의 product_key의 숫자와 customer_id로 group by 된 
product_key 가 같다 = 소비자는 모든 제품을 구매했다!
distinct를 사용한 이유는 제품을 중복해서 구매했을 수 있기 때문에 제품 
구매여부만 있으면 충분하기 때문에 중복값 제거!
with문을 사용했었는데 having절에 (select count(product_key) from product)으로 
간단하게 표현되어서 with문 미사용!