I see. You need to ensure that you get _all_ Person.
For this case, I guess you are right. You would need to first aggregate
the folder per person:
KTable allPersonFolders =
folder.groupBy((...) -> (folder.customerId, ...))
.aggregate(...)
And in a second step, do a left join:
Thanks so much for taking a look. An FK-table-table join is an inner
join which implies there would be no Person entites without associated
Folders. Unfortunately, that's not the case. That lead me to an
attempt of re-keying the Folder topic by each of the three possible
foreign keys in order to
