faster than 98%
\n\nSELECT\n    customer_id\nFROM\n    (SELECT \n        customer_id,\n        COUNT(DISTINCT(product_key)) AS c\n    FROM Customer\n        GROUP BY customer_i
```\n\nSELECT\n    customer_id\nFROM\n    (SELECT \n        customer_id,\n        COUNT(DISTINCT(product_key)) AS c\n    FROM Customer\n        GROUP BY customer_id) A\nWHERE c = (SELECT COUNT(*) FROM product)\n```
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