when the last time patient 3712 was prescribed a medicine via oral route in the last hospital visit?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3712 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.route = 'oral' order by prescriptions.startdate desc limit 1