Oracle:con una relación de uno a muchos, seleccione filas distintas en función de un valor mínimo

Por lo general, uno usa row_number() :

select id, name, visit_date as first_visit_date, reference_number
from (select v.id, p.name, v.visit_date, v.reference_number,
             row_number() over (partition by p.id order by v.visit_date desc) as seqnum
      from visits v join
           patients p
           on v.patient_id p.id
     ) t
where seqnum = 1;