what is the monthly average value of patient 5828's mch?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 5828 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'mch' ) group by strftime('%y-%m',labevents.charttime)