Thank you @mebeingken, your solution worked 
Fortunately, there will not be tons of users in my case so it shouldn’t be too slow.
NB for others facing the same issue:
- Be sure to check “Ignore if empty constraint” (depending on your case)
- This solution doesn’t work if the User fields “List” (here “Activity Area” and “Interests”) are empty in the database (eg User Joe doesn’t have any Interests listed)
(Tried to play around with this, with no luck => mandatory field)
Thanks again Ken for taking the time to reply and for solving this! 