> it turned out that this was technically not allowed.

And thought “no, it turned out this query made no sense and had ill-defined results”.

But the rest of the description is odd:

> In particular, it was not allowed to have a grouped table ordered by a column that is not exposed by the SELECT list of the grouped table.

And I’m wondering if the example query was wrong and they meant:

    SELECT product.product_id, sum(product_part.num)
    FROM product JOIN product_part ON product.product_id = product_part.product_id
    GROUP BY product.product_id, product.product_code
    ORDER BY product.product_code;