SQL problem - Customers Who Bought All Products

Problem

Query

SELECT c.customer_id
FROM Customer AS c
GROUP BY c.customer_id
HAVING COUNT(DISTINCT c.product_key) = (
    SELECT COUNT(*)
    FROM Product
)

Editorial Solution

Approach 1: Count how many products each customer bought

SELECT
  customer_id
FROM
  Customer
GROUP BY
  customer_id
HAVING
  COUNT(DISTINCT product_key) = (
    SELECT
      COUNT(product_key)
    FROM
      Product
  );

Approach 2: Use nested subquery with Cartesian Product - Alternative

Just for learning, do not recommend.

SELECT DISTINCT
  customer_id
FROM
  Customer
WHERE
  customer_id NOT IN (
    SELECT
      customer_id
    FROM
      (
        SELECT DISTINCT
          Customer.customer_id,
          Product.product_key
        FROM
          Customer,
          Product
      ) AS AllPossibleCases
    WHERE
      (customer_id, product_key) NOT IN (
        SELECT
          customer_id,
          product_key
        FROM
          Customer
      )
  );