plutôt alors :

SELECT sum(coalesce(L_CMDCLI.LCQTE,0)) AS CPT, menuder.MDLIBELLE FROM CMDCLI
JOIN L_CMDCLI ON (CMDCLI.COIDCDE = L_CMDCLI.LCIDCDE)
JOIN MENUDER ON MENUDER.MDIDMDER=L_CMDCLI.LCIDSAV...