what is the daily average amount of ph patient 58928 has in their last hospital visit?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 58928 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ph' ) group by strftime('%y-%m-%d',labevents.charttime)