카테고리 별 도서 판매량 집계하기

SELECT A.CATEGORY,
SUM(B.SALES) AS TOTAL_SALES
FROM BOOK A 
JOIN BOOK_SALES B 
ON A.BOOK_ID = B.BOOK_ID
AND MONTH(B.SALES_DATE) = 1
GROUP BY A.CATEGORY
ORDER BY A.CATEGORY;