1) The third request, which you call the "total order price", has an unnecessary subquery and fields. Here's the equivalent:
SELECT
OrderID,
Sum(P.CurrentPrice* OrderItem.Quantity) AS OrderTotal
FROM OrderItem INNER JOIN Product AS P ON OrderItem.ProductID = P.ID
GROUP BY OrderId
2) " ", , . (?) , , []... , , / .
:
SELECT
O.ID, O.DateOf , O.ClientID, O.EmployeeID, O.Desc,
Sum(P.CurrentPrice* OrderItem.Quantity) AS [OrderTotal]
FROM
([Order] As O INNER JOIN OrderItem ON O.ID = OrderItem.OrderID)
INNER JOIN Product AS P ON OrderItem.ProductID = P.ID
GROUP BY
O.ID, O.DateOf , O.ClientID, O.EmployeeID, O.Desc
, , sum() , Group By.
SELECT
O.ID, C.SecondName, E.SecondName,
O.DateOf , O.ClientID, O.EmployeeID, O.Desc,
Sum(P.CurrentPrice* OrderItem.Quantity) AS [OrderTotal]
FROM ((
(Client AS C INNER JOIN [ORDER] AS O ON C.ID = O.ClientID)
INNER JOIN Employee AS E ON E.ID = O.EmployeeID)
INNER JOIN OrderItem ON O.ID = OrderItem.OrderID)
INNER JOIN Product AS P ON OrderItem.ProductID = P.ID
GROUP BY
O.ID, C.SecondName, E.SecondName, O.DateOf,
O.ClientID, O.EmployeeID, O.Desc