what's patient 51497's average value of ptt in 07/2105?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 51497 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ptt' ) and strftime('%y-%m',labevents.charttime) = '2105-07'