so what's the average cholesterol ratio (total/hdl) value of patient 64538 until 67 months ago?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64538 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'cholesterol ratio (total/hdl)' ) and datetime(labevents.charttime) <= datetime(current_time,'-67 month')