Robust + simple answer
sql\nselect customer_id\nfrom Customer \nwhere product_key in (select distinct product_key from Product)\ngroup by customer_id\nhaving count(distinct product_ke
```sql\nselect customer_id\nfrom Customer \nwhere product_key in (select distinct product_key from Product)\ngroup by customer_id\nhaving count(distinct product_key) = (select count(distinct product_key) from product);\n```
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