Cetak nama semua kegiatan dengan jumlah peserta baik maksimum maupun minimum

Anda dapat menggunakan window functions jika versi mysql Anda 8 atau lebih

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);