>Looks like WHERE is executed before SELECT list is build.
>You can move condition into HAVING clause
>

>      ON TABLE1.PART_NO = C_USED.PART_NO 
>  HAVING USED_QUANTITY < 10
>

>Or you can change 'USED_QUANTITY < 10' to
>

>(QTSum IS NULL OR QTSum < 10)
>

Yep, to complete our circle :) Since this is exactly what I suggested in my first message.