CREATE FUNCTION fnGetCustomerCount(@ProductKey AS INT)
RETURNS INT AS
BEGIN
    RETURN (SELECT P.EnglishProductName, count(C.FirstName) AS NoOfCustomers
            FROM Dimproduct AS P
                INNER JOIN FactInternetSales AS F ON P.ProductKey = F.ProductKey
                INNER JOIN DimCustomer AS C ON C.CustomerKey = F.CustomerKey
                WHERE P.ProductKey = @ProductKey
            GROUP BY P.EnglishProductName)
END

Select count(CustomerID), ProductID from Sales