what is the monthly average amount of anion gap patient 69819 has until 1 year ago?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 69819 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'anion gap' ) and datetime(labevents.charttime) <= datetime(current_time,'-1 year') group by strftime('%y-%m',labevents.charttime)