Database Rollup

Indeed! Countless applications in everyday life, e.g. Birthday party -> Add object types Event and Invite; Event holds date and Invites; Invite holds one or more links to Person, along with RSVP status and the number of attending people, e.g., including +1s or children which I don't have as Person objects in my Capacities. How would I know how many people to expect at the party?

Yes. Be able to obtain the sum of a numeric property of an object queried would be very usefull