Hola que tal?, estuve tratando de hacer de la forma que me dijiste pero me da un error en la última línea donde dice as t1.
Código SQL:
Ver originalINSERT INTO Item
SELECT t1.* FROM Item_temp AS t1
LEFT JOIN Item AS t2 ON (t1.PartNumber = t2.PartNumber)
WHERE t2.PartNumber IS NULL
UPDATE Item
SET Price = t1.Price
FROM
(
SELECT * FROM Item_Temp AS t1
LEFT JOIN Item AS t2 ON (t1.PartNumber = t2.PartNumber)
WHERE t2.PartNumber IS NOT NULL
) AS t1 WHERE t1.PartNumber = Item.PartNumber