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;
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