Adding LEFT JOIN causes "multi-part identifier not bound" - sql

Question

Why does the below query fail when I add the second LEFT JOIN statement? The error states "Multi-part identifier T3.ConfigIDx could not be bound". Prior to adding the statement the column T3.ConfigIDx showed in the results. I tried with and with out T3.
I removed some code within the INNER JOINS for brevity.
-- Add CfgDescription to ComponentID include QuantityWH for all components 'Used' or blank found in the order.
-- Attach Line information like Quantity and DiscountRate from the Order using Configuration ID.
SELECT
BDCComponentAttributes.componentID AS ComponentID,
BDCComponentAttributes.Value AS CfgDescription,
CAST (BDC10.Value AS INT) AS QuantityWH,
CfgIDx,
T3.ConfigIDx
FROM BDCComponentAttributes
Left join BDCComponentAttributes BDC10 on BDC10.ComponentID = BDCComponentAttributes.componentID and BDC10.ComponentAttributeName = 'QuantityWH'
Left join OrderDetails on OrderDetails.ConfigurationID = T3.ConfigIDx
INNER JOIN
(
-- Select ComponentID's and their respective CfgDescription for components found in order. This creates derived table T3.
INNER JOIN
(
-- Select Components in the order NOT 'NotUsed'. This creates derived table T2.
INNER JOIN
(
-- Select ConfigurationID's for components of the order. This creates derived table T1.
) AS T1
ON BDCComponents.CfgID = T1.CfgIDx
) AS T2
ON BDCComponentAttributes.ComponentID = T2.ComponentID
WHERE BDCComponentAttributes.ComponentAttributeName = 'PartInSystem' AND ( 'Used' = IsNull(BDCComponentAttributes.Value,'Used') OR BDCComponentAttributes.Value='Used')
) AS T3
ON BDCComponentAttributes.componentID = T3.ComponentID
WHERE BDCComponentAttributes.ComponentAttributeName = 'CfgDescription'
ORDER BY ComponentID

Answer 1

The from clause in your query starts:
FROM BDCComponentAttributes Left join
BDCComponentAttributes BDC10
on BDC10.ComponentID = BDCComponentAttributes.componentID and
BDC10.ComponentAttributeName = 'QuantityWH' Left join
OrderDetails
on OrderDetails.ConfigurationID = T3.ConfigIDx
When a query is compiled, the from clause is interpreted in lexical ordering -- that is, in the same "left-to-right" "top-to-bottom" way that we read. When the symbol T3 is encountered, it is not defined. That is causing your error. SQL does not have "look-ahead" to see that it is defined later in the from clause.
You can fix this by moving the join condition after the definition of T3.