Remplacement de WM_CONCAT par LISTAGG

SELECT mess_val_col1,
       WM_CONCAT(mepj_fichier_nom)     fichiers,
       mess_pers_initiale               resp_envoi,
       mess_dt_envoi                    envoi
  FROM MESSAGE, message_pieces_jointes
 WHERE mess_id = mepj_mess_id(+)
GROUP BY mess_val_col1,
       mess_syta_code,
       mess_pers_initiale,
       mess_dt_envoi

SELECT mess_val_col1,
       fichier,
       mess_pers_initiale               resp_envoi,
       mess_dt_envoi                    envoi
FROM MESSAGE, 
    (SELECT mepj_mess_id, 
            LISTAGG(mepj_fichier_nom, ', ') WITHIN GROUP (ORDER BY mepj_mess_id) AS fichiers
    FROM MESSAGE_PIECES_JOINTES
    GROUP BY mepj_mess_id)
WHERE mess_id = mepj_mess_id(+)
GROUP BY mess_val_col1
        ,mess_syta_code
        ,mess_pers_initiale
        ,mess_dt_envoi
        ,fichiers