https://school.programmers.co.kr/learn/courses/30/lessons/131535
개수 세기 = COUNT()
SELECT COUNT(*) AS USERS
FROM USER_INFO
WHERE JOINED LIKE "2021%" AND 20 <= AGE AND AGE <= 29
year(joined) ="2021"
DATEFORMAT(JOINED, '%Y') = 2021
https://myhappyman.tistory.com/73
AGE BETWEEN 20 AND 29
https://clear-sky-sun.tistory.com/28
더 나은 가독성을 위해 많은 함수를 알아놔야겠다.