ปีนี้มีการจ่าย d5w ให้กับผู้ป่วย 8421 กี่ครั้ง? ไม่จำเป็นต้องใช้ตารางทั้งหมด
how many times d5w has been prescribed to patient 8421 in this year?It is not neccessary to use all the tables.
 chartevents:  row_id [ number]  subject_id [ number]  hadm_id [ number]  icustay_id [ number]  itemid [ number]  charttime [ time]  valuenum [ number]  valueuom [ text] 
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')