since 12/2104, what was the daily minimum value of the pt of patient 31854?
select min(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 31854 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'pt' ) and strftime('%y-%m',labevents.charttime) >= '2104-12' group by strftime('%y-%m-%d',labevents.charttime)