ยาสี่ชนิดใดที่สั่งจ่ายบ่อยที่สุดจนถึงปี 2101?ไม่จำเป็นต้องใช้ตารางทั้งหมด
what were the four medicines most frequently prescribed until 2101?It is not neccessary to use all the tables.
 prescriptions:  row_id [ number]  subject_id [ number]  hadm_id [ number]  startdate [ time]  enddate [ time]  drug [ text]  dose_val_rx [ text]  dose_unit_rx [ text]  route [ text] 
SELECT t1.drug FROM (SELECT prescriptions.drug, DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS c1 FROM prescriptions WHERE STRFTIME('%y', prescriptions.startdate) <= '2101' GROUP BY prescriptions.drug) AS t1 WHERE t1.c1 <= 4