select max(id) as id from patient p
join (select
  max(created_at) as created_at,
  master_patient_id,
  case when master_patient_id is null then id else null end as id
  from patient
  group by master_patient_id, case when master_patient_id is null then id else null end
) s on (s.id = p.id or s.master_patient_id = p.master_patient_id) and s.created_at = p.created_at