what is the monthly minimum value of patient 63849's ph since 98 months ago?
select min(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 63849 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ph' ) and datetime(labevents.charttime) >= datetime(current_time,'-98 month') group by strftime('%y-%m',labevents.charttime)