-- 카테고리 별 -- 도서 판매량 select category CATEGORY, sum(sales) TOTAL_SALES FROM BOOK b inner join BOOK_SALES s using(book_id) where sales_date LIKE '2022-01-%' GROUP BY CATEGORY ORDER BY CATEGORY ASC
book_id가 같음 -> 컬럼명같을땐 using()
https://www.programiz.com/sql/inner-join