1. No. But you can't really do that with straight SQL, either. However, you can sometimes turn it around, by querying on the many side of the relationship, and grouping or sorting the results.
2. Same as above. You have to loop. (It's only three lines of code!) If you want it in one query, just like above, you have to start from the many side.
3. $parent->distinct()->where_related('child', 'id IS NOT NULL')->get();
DMZ is just a wrapper on SQL. If you can't easily do it in SQL, you can't easily do it in DMZ. (Some things are always a little more difficult in DMZ, too.)
