[SQL][프로그래머스] 입양 시각 구하기(2)

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

코딩테스트 연습 - 입양 시각 구하기(2)

SELECT H.HOUR, IFNULL(OUTS.COUNT,0) AS COUNT 
FROM ( 
    SELECT 0 AS HOUR 
    UNION SELECT 1 
    UNION SELECT 2 
    UNION SELECT 3 
    UNION SELECT 4 
    UNION SELECT 5 
    UNION SELECT 6 
    UNION SELECT 7 
    UNION SELECT 8 
    UNION SELECT 9 
    UNION SELECT 10 
    UNION SELECT 11
    UNION SELECT 12
    UNION SELECT 13 
    UNION SELECT 14
    UNION SELECT 15
    UNION SELECT 16 
    UNION SELECT 17 
    UNION SELECT 18
    UNION SELECT 19
    UNION SELECT 20
    UNION SELECT 21
    UNION SELECT 22
    UNION SELECT 23 ) H
LEFT JOIN ( 
    SELECT HOUR(DATETIME) AS HOUR, COUNT(*) AS COUNT 
    FROM ANIMAL_OUTS 
    GROUP BY HOUR) AS OUTS ON H.HOUR = OUTS.HOUR

다른사람의 풀이
set @hour := -1;
SELECT 
    (@hour := @hour + 1) as 'HOUR',
    (select count(*) from animal_outs where hour(datetime) = @hour) as 'COUNT'
from 
    animal_outs
where
    @hour < 23;