count the number of times that patient 87530 has until 20 months ago had gastric nasogastric output.
select count(*) 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 = 87530 ) ) and outputevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'gastric nasogastric' and d_items.linksto = 'outputevents' ) and datetime(outputevents.charttime) <= datetime(current_time,'-20 month')