INNER JOIN product
ON sales_fact.product_id = product.product_id
GROUP BY product_name
returns a list of aggregate sum by product.
But your main query expects only one value due you're grouping by month.
You provide a solution - which I think is correct - but the explanation is not entirely accurate
> SQL Server returns an error indicating that your subquery returns more than one value.
No, if you remove GROUP BY month from the main query, I think you'll get no error!
The subquery returns 1 row - assuming product_id is the PK in products