select q1.* from (
  select m.mes_typos, m.mes_onoma, max(d.diaf_kostos) from diafimisi as d, meso as m where d.diaf_mes_kwdik = m.mes_kwdikos group by  m.mes_typos,m.mes_onoma
) q1 join
(
 select m.mes_typos, max(d.diaf_kostos) from diafimisi as d, meso as m where d.diaf_mes_kwdik = m.mes_kwdikos group by m.mes_typos
) q2 using (mes_typos, max);