For the ORDER BY in grouped table (F868), it's kind of weird. I guess it's a special case with narrow usage.

In the example, product_id is unique (as PK) along with product_code being unique. There's one product_id for each product_code. A Group By on product_id will produce a list of groups where each group has one product_id and one product_code. A subsequent Order By on the groups by the product_code would work. I assume if there're multiple product_code's per product_id, the query won't work.

    CREATE TABLE product (
        product_id int PRIMARY KEY,
        product_name varchar,
        product_code varchar UNIQUE
    );

    SELECT product.product_id, sum(product_part.num) ...
    GROUP BY product.product_id
    ORDER BY product.product_code;