ยาที่ผู้ป่วย 21079 จ่ายสองครั้งคืออะไร?ไม่จำเป็นต้องใช้ตารางทั้งหมด
what is the drug that patient 21079 was prescribed two times?It is not neccessary to use all the tables.
 outputevents:  row_id [ number]  subject_id [ number]  hadm_id [ number]  icustay_id [ number]  charttime [ time]  itemid [ number]  value [ number] 
SELECT t1.drug FROM (SELECT prescriptions.drug, COUNT(prescriptions.startdate) AS c1 FROM prescriptions WHERE prescriptions.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 21079) GROUP BY prescriptions.drug) AS t1 WHERE t1.c1 = 2