ค้นหารหัสของผู้สอนที่สอนชั้นเรียนในฤดูใบไม้ร่วงปี 2009 แต่ไม่ใช่ในฤดูใบไม้ผลิปี 2010
Find the id of instructors who taught a class in Fall 2009 but not in Spring 2010.
df_teaches = pd.DataFrame(columns=['id', 'semester', 'YEAR'])
df_teaches[(df_teaches['semester'] == 'Fall') & (df_teaches['YEAR'] == '2009')]['id'].difference(df_teaches[(df_teaches['semester'] == 'Spring') & (df_teaches['YEAR'] == '2010')]['id'])