ครั้งแรกที่ผู้ป่วย 51200 ได้รับการทดสอบ ptt ในช่วงเดือนที่ผ่านมาคือเมื่อใด?ไม่จำเป็นต้องใช้ตารางทั้งหมด
when was the first time patient 51200 was tested for ptt during the previous month?It is not neccessary to use all the tables.
 labevents:  row_id [ number]  subject_id [ number]  hadm_id [ number]  itemid [ number]  charttime [ time]  valuenum [ number]  valueuom [ text] 
SELECT labevents.charttime FROM labevents WHERE labevents.itemid IN (SELECT d_labitems.itemid FROM d_labitems WHERE d_labitems.label = 'ptt') AND labevents.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 51200) AND DATETIME(labevents.charttime, 'start of month') = DATETIME(CURRENT_TIME(), 'start of month', '-1 month') ORDER BY labevents.charttime LIMIT 1