when was the first time that patient 64538 was prescribed a medication until 11/2103 via oral?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64538 ) and prescriptions.route = 'oral' and strftime('%y-%m',prescriptions.startdate) <= '2103-11' order by prescriptions.startdate asc limit 1