how many times d5w has been prescribed to patient 8421 in this year?
select count(*) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 8421 ) and prescriptions.drug = 'd5w' and datetime(prescriptions.startdate,'start of year') = datetime(current_time,'start of year','-0 year')