what was the maximum value of patient 52456's free calcium last month?
select max(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 52456 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'free calcium' ) and datetime(labevents.charttime,'start of month') = datetime(current_time,'start of month','-1 month')