SELECT distinct
e.casaEditriceId,
e.user, 
e.password, 
e.numeroBadge, 
e.denominazione, 
'' as emailAssistente
FROM tbl_editori e
/* LEFT JOIN tbl_ospiti o on o.casaEditriceId=e.casaEditriceId
 where COALESCE(o.emailAssistente, '') != '' */
ORDER BY e.denominazione /*, o.emailAssistente */