SELECT S.Name+' '+S.Surname AS FullName,Quantity_Seller
FROM Product AS P,Seller AS S
WHERE S.id=P.id
GROUP BY S.Name,S.Surname,P.Quantity_Sell

SELECT S.Name+' '+S.Surname AS FullName, SUM(Quantity_Seller) AS Total
FROM Product AS P,Seller AS S
WHERE S.id=P.id_Seller
GROUP BY S.Name,S.Surname

SELECT P.*
FROM Product AS P,Seller AS S
WHERE S.id=P.id_Seller 
AND S.Name = 'Владислав' AND S.Surname = 'Дубцов'