[프로그래머스] 입양 시각 구하기(2)

문제 설명

ANIMAL_OUTS 테이블은 동물 보호소에서 입양 보낸 동물의 정보를 담은 테이블입니다. ANIMAL_OUTS 테이블 구조는 다음과 같으며, ANIMAL_ID, ANIMAL_TYPE, DATETIME, NAME, SEX_UPON_OUTCOME는 각각 동물의 아이디, 생물 종, 입양일, 이름, 성별 및 중성화 여부를 나타냅니다.

NAMETYPENULLABLE

ANIMAL_ID	VARCHAR(N)	FALSE
ANIMAL_TYPE	VARCHAR(N)	FALSE
DATETIME	DATETIME	FALSE
NAME	VARCHAR(N)	TRUE
SEX_UPON_OUTCOME	VARCHAR(N)	FALSE

보호소에서는 몇 시에 입양이 가장 활발하게 일어나는지 알아보려 합니다. 0시부터 23시까지, 각 시간대별로 입양이 몇 건이나 발생했는지 조회하는 SQL문을 작성해주세요. 이때 결과는 시간대 순으로 정렬해야 합니다.

예시

SQL문을 실행하면 다음과 같이 나와야 합니다.

HOURCOUNT

0	0
1	0
2	0
3	0
4	0
5	0
6	0
7	3
8	1
9	1
10	2
11	13
12	10
13	14
14	9
15	7
16	10
17	12
18	16
19	2
20	0
21	0
22	0
23	0

---

본 문제는 Kaggle의 "Austin Animal Center Shelter Intakes and Outcomes"에서 제공하는 데이터를 사용하였으며 ODbL의 적용을 받습니다.

SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 0 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 0 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 1 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 1 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 2 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 2 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 3 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 3 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 4 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 4 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 5 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 5 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 6 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 6 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 7 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 7 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 8 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 8 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 9 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 9 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 10 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 10 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 11 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 11 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 12 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 12 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 13 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 13 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 14 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 14 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 15 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 15 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 16 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 16 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 17 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 17 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 18 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 18 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 19 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 19 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 20 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 20 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 21 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 21 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 22 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 22 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR
UNION ALL
SELECT A.HOUR, SUM(A.COUNT) AS COUNT
  FROM (
    SELECT 23 AS HOUR
        ,(
            CASE WHEN DATE_FORMAT(T1.DATETIME,'%H')  = 23 THEN 1 ELSE 0 END
        ) AS COUNT
 
        FROM ANIMAL_OUTS AS T1
  ) AS A
GROUP BY A.HOUR

UNION ALL로 하나하나 이어 붙였는데 재귀구문이라는게 있었다.

재귀구문 사용시 훨씬 간단하게 정리할 수 있음

WITH RECURSIVE REC (HOUR) AS (
    SELECT  0
    UNION ALL
    SELECT HOUR+1 FROM REC WHERE HOUR < 23
)
SELECT HOUR
      , IFNULL(COUNT,0) AS COUNT
FROM REC 
    LEFT JOIN (
        SELECT HOUR(DATETIME) as HOUR
             , COUNT(1) AS COUNT
        FROM ANIMAL_OUTS 
        GROUP BY HOUR(DATETIME)
    ) tmp 
    USING (HOUR)