~~별
이라는 단어가 있으면, 그것이 GROUP BY
이다![틀린 코드]
SELECT A.AUTHOR_ID, AUTHOR_NAME, CATEGORY, PRICE *sum(SALES) as TOTAL_SALES
from BOOK_SALES S
join BOOK B on B.BOOK_ID = S.BOOK_ID
join AUTHOR A on A.AUTHOR_ID = B.AUTHOR_ID
where YEAR(SALES_DATE) = 2022 and MONTH(SALES_DATE) = 01
group by AUTHOR_NAME, CATEGORY
order by B.AUTHOR_ID, CATEGORY desc
[맞는 코드]
SELECT A.AUTHOR_ID, AUTHOR_NAME, CATEGORY, sum(PRICE * SALES) as TOTAL_SALES
from BOOK_SALES S
join BOOK B on B.BOOK_ID = S.BOOK_ID
join AUTHOR A on A.AUTHOR_ID = B.AUTHOR_ID
where YEAR(SALES_DATE) = 2022 and MONTH(SALES_DATE) = 01
group by AUTHOR_NAME, CATEGORY
order by B.AUTHOR_ID, CATEGORY desc
https://school.programmers.co.kr/questions/41330?referer=collection-of-questions