EASY MYSQL
\tSELECT customer_id\n\tFROM Customer\n\tGROUP BY customer_id\n\tHAVING count(DISTINCT product_key) = (SELECT count(*) FROM Product)
\tSELECT customer_id\n\tFROM Customer\n\tGROUP BY customer_id\n\tHAVING count(DISTINCT product_key) = (SELECT count(*) FROM Product)
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