I'm not clear yet what you are trying to achieve.
One interpretation of your question would be that you wish to group by
category ID. Another could be that you want to find jobnumbers that fall
into a (pre-selected) category ID.
Have you tried creating a new query that includes your conditions
(selection), then using that query as the source for your combo box?
--
Regards
Jeff Boyce
Microsoft Office/Access MVP
Microsoft IT Academy Program Mentor
http://microsoftitacademy.com/
Microsoft Registered Partner
https://partner.microsoft.com/