on 07/19/2104, what was first value of sao2 for patient 2004?
select 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 = 2004 ) ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'sao2' and d_items.linksto = 'chartevents' ) and strftime('%y-%m-%d',chartevents.charttime) = '2104-07-19' order by chartevents.charttime asc limit 1