calculate the total amount of gastric nasogastric of patient 1272 since 05/06/2102.
select sum(outputevents.value) from outputevents where outputevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1272 ) ) and outputevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'gastric nasogastric' and d_items.linksto = 'outputevents' ) and strftime('%y-%m-%d',outputevents.charttime) >= '2102-05-06'