MAX(CASE WHEN rn = 1 THEN 'K' END)
|| MAX(CASE WHEN rn = 2 THEN 'W' ELSE '' END)
|| MAX(CASE WHEN rn = 3 THEN 'WF' ELSE '' END)
(SELECT ITEM_ID,SELLER_ID, ROW_NUMBER()OVER(PARTITION BY ITEM_ID ORDER BY SELLER_ID ASC) AS RN FROM ITEMSELLER)A
GROUP BY 1,2;
2) UPDATE KW TO WK AND KWWF TO WKWF , as i think they will come in correct order.