select top 1 hobby.*, floor((sysdate - students_hobby.d_start) / 30)
from students
  join students_hobby on students_hobby.n_z = students.n_z
  join hobby on hobby.h_name = students_hobby.h_name
where
  students.n_z = 123 -- номер зачетки
  and students.n_gr = '123' -- номер группы
  and students_hobby.d_finish is null
order by (students_hobby.d_finish - students_hobby.d_start)