create trigger on northwind datase .. please help

I found a temporary solution

create trigger tr_orderTotalPrice on Orders for insert 
as
Begin
select od.unitprice, od.Quantity, 
       Discount=CONVERT(int, Discount * 100), 
       TotalPruductPrice =round(CONVERT(money, Quantity * (1 - Discount *100) * Od.UnitPrice),2)
       into #TotalPruductPrice
from orders o, [Order Details] od, inserted i
where i.orderID = o.orderID
and o.orderID = od.orderID

if (select sum(TotalPruductPrice) from #TotalPruductPrice) > 20000
      BEGIN
                EXEC msdb.dbo.sp_send_dbmail
                        @profile_name = 'AdventureWorks2012 Administrator',
                        @recipients = 'danw@Adventure-Works.com',
                        @body = 'Don''t forget to print a report for the sales force.',
                        @subject = 'Reminder';
                END
End

but in this case I must had inserted values into Order Details table before I insert to Order table

but I can't of course because there is relationship between the two tables

so I have to use the following code to add values to order details table

ALTER TABLE "Order Details" NOCHECK CONSTRAINT ALL
go
INSERT "Order Details" VALUES('','','','','')