ค้นหาชื่อครูที่สอนชั้นประถมศึกษาปีที่ 1
Find the first names of the teachers that teach first grade.
df1 = pd.DataFrame(columns=['classroom']); df2 = pd.DataFrame(columns=['firstname', 'classroom'])
df2.loc[df2['classroom'].isin(df1.loc[df1['grade'] == '1', 'classroom']), 'firstname'].unique()