Print de naam van alle activiteiten met geen maximum of minimum aantal deelnemers

U kunt window functions . gebruiken als uw mysql-versie 8 of hoger is

select activity from (select activity, count(*) as cnt,
                             max(count(*)) over () as maximum_cnt,
                             min(count(*)) over () as minimum_cnt
                        from friends group by activity) mytable
where cnt not in (maximum_cnt, minimum_cnt);