calculate the number of patients who have had a skin scrapings microbiology test since 1 year ago.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'skin scrapings' and datetime(microbiologyevents.charttime) >= datetime(current_time,'-1 year') )