Загрузка данных

SELECT
    Partners.PartnerID,
    Partners.Название,
    Sum(PartnerProducts.Количество) AS ОбщееКоличество,
    IIf(Sum([Количество])<10000,0,
        IIf(Sum([Количество])<50000,5,
            IIf(Sum([Количество])<300000,10,15))) AS Скидка
FROM
    Partners
    INNER JOIN PartnerProducts
        ON Partners.PartnerID = PartnerProducts.PartnerID
GROUP BY
    Partners.PartnerID,
    Partners.Название;