Nz(A.Attachments,Nz(B.Attachments,C.Attachments))
Thank for the advice but like you predicted, it did not work.
Unfortunately since SharePoint has a limit of 5000 items per list, so we are forced to move (archive) student files automatically using Microsoft Flow.
It just would be nice if all lists
A [students]
B [students archive 2-4 years old]
C [students archive 4-6 years old]
would all be searchable from one Form in Access. Which is, thanks to UNION ALL and LEFT JOIN.
But if a particular student is in B but the Access Form's control source is A.Attachments. It'll come back with an error, obviously because the student's attachments are in the B.Attachmetns column not int the A.Attachments column
I tried "=Nz(A.Attachments,Nz(B.Attachments,C.Attachments)) " in the Access Form's control source as well, it did not work.
I'll look into how running append queries to a single table works. Thanks for the suggestion.