-- answer SELECT USER_ID, PRODUCT_ID FROM ONLINE_SALE GROUP BY USER_ID, PRODUCT_ID HAVING COUNT(*) > 1 ORDER BY USER_ID, PRODUCT_ID DESC
USER_ID, PRODUCT_ID가 중복되어야 하므로 GROUP BY 재구매했다면 구매기록이 1 이상일테니 HAVING COUNT(*) > 1
GROUP BY
HAVING COUNT(*) > 1