Gehen tut fast alles, nur eine Frage der Schlüssel:

create view MyView as
with
TempTel as
(select KDNR, Min(Telefon) as Telefon
from TelDatei
group by Kdnr)
,
TempMail as
(Select Kdnr, Min(Mail) as Mail
from MailDatei
Group by Kdnr)
Select Kdnr, coalesce(Telefon, ' ') as Telefon, coalesce(Mail, ' ') as Mail
from Kunden a
left join TempTel t on a.kdnr=t.kdnr
left join TempMail m on a.kdnr=m.kdnr

Coalesce ist erforderlich um NULL, also kein Telefon und keine Mail auszuschließen.