Well that would explain it then!
I read that page and came up with:
FROM new_npt LEFT JOIN aajos_chronoforms_register ON new_npt.npt_code = aajos_chronoforms_register.npt_val
WHERE aajos_chronoforms_register.npt_val IS NULL;
Im not sure where to put the where aajos_chronoforms_register.uid=63 bit, it can't go on the end where clause because that column only contains nulls.