SELECT
GBN
,REPLACE(WM_CONCAT(ITEM_CD),',','')
FROM
(
SELECT 'X' GBN,'A01' ITEM_CD FROM DUAL UNION ALL
SELECT 'X' GBN,'A02' ITEM_CD FROM DUAL UNION ALL
SELECT 'X' GBN,'B02' ITEM_CD FROM DUAL UNION ALL
SELECT 'Y' GBN,'B01' ITEM_CD FROM DUAL UNION ALL
SELECT 'Y' GBN,'B02' ITEM_CD FROM DUAL UNION ALL
SELECT 'Z' GBN,'G01' ITEM_CD FROM DUAL UNION ALL
SELECT 'Z' GBN,'A02' ITEM_CD FROM DUAL UNION ALL
SELECT 'Z' GBN,'B02' ITEM_CD FROM DUAL
)
GROUP BY GBN