what is the monthly minimum lactate dehydrogenase (ld) level of patient 14397 since 03/2104?
select min(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 14397 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'lactate dehydrogenase (ld)' ) and strftime('%y-%m',labevents.charttime) >= '2104-03' group by strftime('%y-%m',labevents.charttime)