Create Table [Order]
(
   OrderID BigInt Identity(1,1) Primary Key,
   OrderTotal Decimal(22,2) Not Null
)

Create Table [OrderDetail]
(
   OrderDetailID BigInt Identity(1,1) Primary Key,
   Total Decimal(22,2) Not Null,
   OrderID BigInt Not Null,
   Constraint [FK_OrderDetail-OrderID] Foreign Key [OrderID] References [Order](Id)
      ON DELETE CASCADE
)

Create Table [Order]
(
   Id BigInt Identity(1,1) Primary Key,
   OrderTotal Decimal(22,2) Not Null
)

Create Table [OrderDetail]
(
   Id BigInt Identity(1,1) Primary Key,
   OrderDetailTotal Decimal(22,2) Not Null,
   OrderID BigInt Not Null,
   Constraint [FK_OrderDetail-OrderID] Foreign Key [OrderID] References [Order](Id)
      ON DELETE CASCADE
)

Create Table [Orders]
(
   OrderId BigInt Identity(1,1) Primary Key
)

go

Create Table [OrderDetails]
(
   OrderDetailsId BigInt Identity(1,1) Primary Key,
   OrderDetailTotal Decimal(22,2) Not Null,
   OrderID BigInt Not Null,
   Constraint [FK_OrderDetail-OrderID] Foreign Key (OrderID) References [Orders](OrderId)
      ON DELETE CASCADE
)

go

create view OrderSummary
as 
	select o.OrderID, SUM(OrderDetailTotal) as OrderTotal
	from Orders o
	join OrderDetails od on od.OrderID = o.OrderID
	group by o.OrderID

go

select * from OrderSummary

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

if EXISTS(select * from inserted) and NOT EXISTS(select * from deleted)
--do your inserted stuff here

if EXISTS(select * from deleted) and NOT EXISTS(select * from inserted)
--do your deleted stuff here

if EXISTS(select * from deleted) and EXISTS(select * from inserted)
--do your update stuff here