SQL Server 2019 - Group

SELECT (
SELECT CONCAT(E.LastName, + ' ' ,+ E.FirstName) FROM dbo.Employees AS E WHERE E.EmployeeID = O.EmployeeID
) AS Seller,
(
SELECT C.ContactName
FROM dbo.Customers AS C
WHERE C.CustomerID = O.CustomerID
) AS Customer
,Count (*) AS Amount
FROM dbo.Orders AS O
GROUP BY CUBE (O.EmployeeID, O.CustomerID)
ORDER BY COUNT(O.OrderID) DESC;
GO