Merge statement for stocks using three tables - sql-server

Question

Here is the SQL Query:
MERGE tblProductsSold
USING tblOrders on tblOrders.OrderID = tblProductsSold.txtOrderID
WHEN NOT MATCHED THEN
Insert ( txtOrderID, txtOrderdate, txtPartno, txtQty)
values
(SELECT tblItemsOnOrder.txtOrderID,
tblOrders.txtDateTime,
tblItemsOnOrder.txtPartNO,
tblItemsOnOrder.txtQTY
FROM tblOrders INNER JOIN tblItemsOnOrder
ON tblOrders.OrderID = tblItemsOnOrder.txtOrderID
WHERE tblOrders.txtIsConfirmed = '1'
)
OUTPUT $action ;
Desired Result: need to import orders with Products that are not already in the tblProductsSold table

Answer 1

You cannot approach it like you are doing it right now.
The MERGE statement merges two tables - the two tables you define in the header - the source table and the target table.
Right now, you're using tblOrders as your source, and tblProducts as your target. That alone seems odd - you're trying to merge orders into products? Doesn't seem very fitting...
Once you've defined your source and target table - you stat comparing which rows from the source are present in the target (or not). If a given row from your source is not present in the target - then you can insert its values into the target table.
But that only works for direct column values from the source table! You cannot go out and do subqueries into other tables as you're trying to do!
So I believe what you really should do is this:
as your source - have a view that lists the products found in your orders - the products (not the orders per se)
then compare your Products table to this view - if your orders happen to have any products that aren't present in the base Products table - insert them.
So you'd need something like:
MERGE tblProductsSold AS Target
USING (SELECT tblItemsOnOrder.txtOrderID, tblOrders.txtDateTime,
tblItemsOnOrder.txtPartNO, tblItemsOnOrder.txtQty
FROM tblOrders
INNER JOIN tblItemsOnOrder ON tblOrders.OrderID = tblItemsOnOrder.txtOrderID
WHERE tblOrders.txtIsConfirmed = '1') AS Source
ON Source.OrderID = Target.txtOrderID
WHEN NOT MATCHED THEN
INSERT (txtOrderID, txtOrderdate, txtPartno, txtQty)
VALUES (Source.OrderID, Source.txtDateTime, Source.txtPartNo, Source.txtQty)
OUTPUT $action ;