gistfile1.txt

-- SQLクイズへの回答 https://github.com/neumann-tokyo/sql-quiz/blob/main/store/question.md

SELECT r.item_name,
       r.weeknum,
       COALESCE(s.price, 0) AS lastweek_price,
       r.price,
       CASE
         WHEN COALESCE(s.price, 0) < r.price THEN '/'
         WHEN COALESCE(s.price, 0) > r.price THEN '\'
         ELSE '='
       END AS mark
FROM   reports r
       LEFT JOIN reports s
              ON r.item_name = s.item_name
                 AND r.weeknum - 1 = s.weeknum
ORDER  BY r.item_name,
          r.weeknum;