select tb.* from

(

select sum(nr_locuitori)nrloc,t.id_tara,t.nume_tara,j.nume_judet

from judet j join localitate l on j.id_judet=l.id_judet

join tara t on t.id_tara=j.id_tara

group by nume_judet,t.nume_tara,t.id_tara

) tb

join tara t2 on tb.id_tara=t2.id_tara and

nrloc=( select max(nrloc) from (select sum(nr_locuitori)nrloc,j.id_tara

                                       from judet j join localitate l on j.id_judet=l.id_judet

                                       where j.id_tara=t2.id_tara

                                       group by nume_judet,j.id_tara

                                       )tab2)