SELECT Name, ROUND(SUM(Quantity * UnitCost) * 100/TotalSum,2) as Procent
FROM product
INNER JOIN orderdetails ON orderdetails.ProductId = product.Id
INNER JOIN (
    SELECT SUM(Quantity * UnitCost) as TotalSum,
    CategoryId
    FROM orderdetails
    INNER JOIN product ON orderdetails.ProductId = product.Id
    GROUP BY CategoryId) AS Summary ON
    product.CategoryId = Summary.CategoryId
    GROUP BY product.Id,Name,TotalSum;