select * from
(select field_a,field_b,max(xx)
from tableX
group by field_a,field_b) t1,tablex t2
where t1.field_a=t2.field_a and t1.field_b=t2.field_b and t1.max_xx=t2.xx
(select field_a,field_b,max(xx)
from tableX
group by field_a,field_b) t1,tablex t2
where t1.field_a=t2.field_a and t1.field_b=t2.field_b and t1.max_xx=t2.xx
from (select rowid,field_a,field_b,xx,
rank() over (partition by field_a,field_b order by xx desc ) rk from tableX
) a
where rk=1