calculate the number of patients who received a lab test of magnesium, urine in this year.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select labevents.hadm_id from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'magnesium, urine' ) and datetime(labevents.charttime,'start of year') = datetime(current_time,'start of year','-0 year') )