select grupa,an from (
select t1.cnt1/t2.cnt2, t1.grupa, t1.an from
(select count(*) as cnt1, s1.grupa, s1.an from
studenti s1 join prieteni p on s1.id=p.id_student1 join studenti s2
on s2.id = p.id_student2 and s1.grupa=s2.grupa and s1.an=s2.an
group by s1.grupa, s1.an) t1
join
(select count(*) as cnt2, grupa, an from studenti
group by grupa,an) t2
on t1.an=t2.an and t1.grupa=t2.grupa order by 1 desc) where rownum=1;