ค้นหารหัสของผู้สมัครที่มีอีเมลคือ stanley.monahan@example.org
Find id of the candidate whose email is stanley.monahan@example.org?
candidates = pd.DataFrame(columns=['candidate_id']); people = pd.DataFrame(columns=['person_id', 'email_address'])
candidates.merge(people[people['email_address'] == 'stanley.monahan@example.org'], left_on='candidate_id', right_on='person_id')['candidate_id'].iloc[0]