[프로그래머스 SQL 고득점 kit(oracle)] GROUP BY - 입양 시각 구하기(2)(level4)

https://programmers.co.kr/learn/courses/30/lessons/59413

프로그래머스

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SELECT A.HOUR, NVL(B.COUNT, 0) AS COUNT
FROM  
    (SELECT LEVEL -1 AS HOUR
    FROM DUAL
    CONNECT BY LEVEL <= 24) A,
    (SELECT TO_CHAR(DATETIME, 'HH24') AS HOUR, COUNT(*) AS COUNT
    FROM ANIMAL_OUTS
    GROUP BY TO_CHAR(DATETIME, 'HH24')) B
WHERE A.HOUR = B.HOUR(+)
ORDER BY A.HOUR;