so what's the average urea nitrogen, urine value of patient 12775 on this hospital encounter?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 12775 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'urea nitrogen, urine' )