I don't like joins, so here you go: 4-liner HAVING() solution
select customer_id\nfrom Customer\ngroup by 1\nhaving count(distinct product_key) = (select count(distinct product_key)  from Product)
select customer_id\nfrom Customer\ngroup by 1\nhaving count(distinct product_key) = (select count(distinct product_key)  from Product)
[]