Alvin,

You should use only one of the tables in each select in union: OrderItems in the first and ServiceParts in the second. Also, you don't need DISTINCT because UNION does implicit DISTINCT on all rows.

SELECT oi.LineNum 
	FROM ORDERS o
	 INNER JOIN ROOM r ON o.OrderID = r.OrderID
	 INNER JOIN GlobalOptions go On go.RoomID = r.RoomID
	 LEFT JOIN OrderItems oi ON oi.RoomID = r.RoomID
	WHERE o.Number = 'TDT-777'
UNION
	SELECT sp.LineNum 
	FROM ORDERS o
	 INNER JOIN ROOM r ON o.OrderID = r.OrderID
	 INNER JOIN GlobalOptions go On go.RoomID = r.RoomID
	 LEFT JOIN ServiceParts sp ON sp.RoomID = r.RoomID
	WHERE o.Number = 'TDT-777'

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