what is the first value of the sodium of patient 8116 since 03/2102?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 8116 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'sodium' ) and strftime('%y-%m',labevents.charttime) >= '2102-03' order by labevents.charttime asc limit 1