11/14 크기가 작은 부분 문자열
https://school.programmers.co.kr/learn/courses/30/lessons/147355
https://school.programmers.co.kr/learn/courses/30/lessons/147355
https://school.programmers.co.kr/learn/courses/30/lessons/160586
판다스 DataFrame에서 null 유무 검사하는 법
https://school.programmers.co.kr/learn/courses/30/lessons/138477
https://school.programmers.co.kr/learn/courses/30/lessons/142086