I finally decided to use CTE in this trigger after reading Sql Documentation and googling:

CREATE TRIGGER [trUpdateOrderTotal]

ON [dbo].[OrderDetail]

FOR DELETE, INSERT, UPDATE

AS

BEGIN

Declare @evt Int = 0;

SET NOCOUNT ON

IF(EXISTS(SELECT * FROM inserted))

Set @evt = @evt + 1;

IF(EXISTS(SELECT * FROM deleted))

Set @evt = @evt + 2;

IF(@Evt = 1) -- inserted

begin

With SubTotal(OrderID, SubTotal) As

(

SELECT OD.OrderID, SUM(COALESCE(OD.Total, 0)) As SubTotal FROM OrderDetail OD

WHERE OD.OrderID IN (SELECT Distinct(OrderID) From inserted)

GROUP BY OD.OrderID

)

UPDATE O SET O.OrderTotal = S.SubTotal

FROM [Order] O JOIN SubTotal S ON S.OrderID = O.OrderID

end

if(@Evt = 2) -- deleted

begin

With SubTotal(OrderID, SubTotal) As

(

SELECT OD.OrderID, SUM(COALESCE(OD.Total, 0)) As SubTotal FROM OrderDetail OD

WHERE OD.OrderID IN (SELECT Distinct(OrderID) From deleted)

GROUP BY OD.OrderID

)

UPDATE O SET O.OrderTotal = S.SubTotal

FROM [Order] O JOIN SubTotal S ON S.OrderID = O.OrderID

end

if(@Evt = 3) -- updated

begin

With SubTotal(OrderID, SubTotal) As

(

SELECT OD.OrderID, SUM(COALESCE(OD.Total, 0)) As SubTotal FROM OrderDetail OD

WHERE OD.OrderID IN (SELECT Distinct(OrderID) From inserted WHERE COLUMNS_UPDATED(Total) = 1)

GROUP BY OD.OrderID

)

UPDATE O SET O.OrderTotal = S.SubTotal

FROM [Order] O JOIN SubTotal S ON S.OrderID = O.OrderID

end

END