what is the maximum magnesium value in 03/last year for patient 3911?
select max(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3911 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'magnesium' ) and datetime(labevents.charttime,'start of year') = datetime(current_time,'start of year','-1 year') and strftime('%m',labevents.charttime) = '03'