what was patient 25815's minimum sao2,?
select min(chartevents.valuenum) from chartevents where chartevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 25815 ) ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'sao2' and d_items.linksto = 'chartevents' )