SELECT * FROM (
SELECT CNB_FILIAL, CNB_NUMERO, CNB_CONTRA, CNB_REVISA, CNB_PRODUT, CNB_ITEM CNBITEM, NVL((SELECT PHM_ITEM
    FROM PHM000 PHM
    WHERE PHM_FILIAL = CNB_FILIAL
    AND PHM_CONTRA = CNB_CONTRA
    AND PHM_REVISA = CNB_REVISA
    AND PHM_NUMERO = CNB_NUMERO
    AND PHM_PRODUT = CNB_PRODUT
    AND PHM_CMPFAT = '12/2018'
    AND PHM.D_E_L_E_T_ = ' ' OFFSET 0 ROWS FETCH NEXT 1 ROWS ONLY),'NAO ENCONTRADO') AS PHMITEM
    FROM CNB000 CNB
    WHERE CNB_FILIAL = '  '
    AND CNB_CONTRA IN (SELECT DISTINCT PHM_CONTRA FROM PHM000 PHM WHERE PHM.D_E_L_E_T_ = ' ' AND PHM_CMPFAT = '12/2018')
    AND CNB_XBILLI = '1'
    AND CNB.D_E_L_E_T_ = ' ' ) TRB
WHERE CNBITEM <> PHMITEM