SQL SERVER - easiest solution
\n\n# Code\n\n/* Write your T-SQL query statement below */\nDECLARE @counter INT;\nSELECT @counter = COUNT(* ) FROM product;\n\nselect customer_id \nfrom custom
\n\n# Code\n```\n/* Write your T-SQL query statement below */\nDECLARE @counter INT;\nSELECT @counter = COUNT(* ) FROM product;\n\nselect customer_id \nfrom customer\ngroup by customer_id\nhaving count(distinct product_key) = @counter\n```
['MS SQL Server']