SELECT
    COALESCE(IT_M.ITEM_CODE, IT_H.ITEM_CODE) AS CODE
    , COALESCE(IT_M.ITEM_NAME, IT_H.ITEM_NAME) AS NAME
    , CASE
        WHEN IT_H.USER_UPDATE_DATETIME IS NULL
            THEN 'ADDED'
        WHEN IT_M.USER_UPDATE_DATETIME IS NULL
            THEN 'DELETED'
        WHEN IT_M.USER_UPDATE_DATETIME <> IT_H.USER_UPDATE_DATETIME
            THEN 'UPDATED'
        END AS COMP_RSLT
FROM
    ITEM AS IT_M
    FULL OUTER JOIN ITEM_HISTORY AS IT_H
        ON IT_M.ITEM_CODE = IT_H.ITEM_CODE
WHERE
    -- IS NOTを使うとNULL値も比較可能となる
    IT_M.USER_UPDATE_DATETIME IS NOT IT_H.USER_UPDATE_DATETIME
ORDER BY
    CODE DESC;