If you have id without gaps go, just joinin to the sample id-1. If missing, it is a little more complicated the request is.
So if the ID in order, for example:
SELECT t2.partnum p2, t1.partnum p1, t2.partnum - t1.partnum res FROM nspcl t1 Nspcl INNER JOIN t2 ON t1.id = (t2.id - 1)