ผู้แทนคนไหนมาจากเทศมณฑลที่มีประชากรน้อยกว่า 100,000 คน
Which delegates are from counties with population smaller than 100000?
df_election = pd.DataFrame(columns=['Delegate', 'District']); df_county = pd.DataFrame(columns=['County_id', 'Population'])
df_election.merge(df_county[df_county['Population'] < 100000], left_on='District', right_on='County_id')['Delegate']