the first time patient 20898 was prescribed bisacodyl in 12/2105?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 20898 ) and prescriptions.drug = 'bisacodyl' and strftime('%y-%m',prescriptions.startdate) = '2105-12' order by prescriptions.startdate asc limit 1