Customers who bought every product

Tables:

product(id int primary key)

purchases(customer_id int, product_id int) (a customer can buy the same product many times; product_id may refer to a product that is no longer in product)

Return the ids of customers who bought every product in product at least once. If product is empty, return no rows.

Output column: customer_id.

CREATE TABLE product (
  id int PRIMARY KEY
);
CREATE TABLE purchases (
  customer_id int NOT NULL,
  product_id int NOT NULL
);