[SQL/ORACLE] 계층적 쿼리(hierarchical Queries) (ING)

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

프로그래머스

SELECT 
    A.HOUR,
    count(B.TIME) count
from (select level-1 HOUR from dual connect by level <25) A,
    (select to_char(datetime,'HH24') TIME from animal_outs) B
where A.HOUR = B.TIME(+)
group by A.HOUR
order by A.HOUR

CONNECT BY LEVEL : 연속된 숫자를 조회할 때 활용하는 계층 쿼리

"SELECT LEVEL-1 HOUR FROM DUAL CONNECT BY LEVEL < 25"는 0부터 23까지 순차적으로 출력하기 위한 쿼리

 

 

https://docs.oracle.com/cd/E11882_01/server.112/e41084/queries003.htm#SQLRF52332

Hierarchical Queries