https://programmers.co.kr/learn/courses/30/lessons/59412
SELECT HOUR(DATETIME) AS HOUR, COUNT(*) AS COUNT FROM ANIMAL_OUTS WHERE HOUR(DATETIME) >= 9 AND HOUR(DATETIME) < 20 GROUP BY HOUR(DATETIME) ORDER BY HOUR(DATETIME);
날짜, 시분초 다 나오는 DATETIME을
HOUR(DATETIME)함으로써 시간만 분리가 가능하다는 점 !