Two things:

1) Don't mix SQL-89 (implicit JOIN) and SQL-92 (explicit JOIN).
2) Referring to KO.TYP in the WHERE clause in reality changes the left join to an inner join.

So, assuming you really want a left join, change to:

SELECT DISTINCT
PR.SUCHBEGRIFF as Liegenschaft,
AD.VORNAME,
AD.NAME1,
KO.NUMMER,
KO.TYP
FROM ADRESSE ad
JOIN ADRESSE_ADRESSKATEGORIE aa on AA.ADRESSE_ID = AD.ID
JOIN ADRESSKATEGORIE ak on AK.ID = AA.ADRESSKATEGORIE_ID
JOIN VERTEILEREINTRAG ve on VE.ADRESSE_ID = AD.ID
JOIN PROJEKT pr on VE.VERTEILER_ID = PR.BETEILIGTEVERTEILER_ID
left join KOMMUNIKATION ko on AD.ID = KO.ADRESSE_ID and KO.TYP = 'Privat'
WHERE
AK.SUCHBEGRIFF = 'Hauswart'

And see if that eliminates your problem.

HTH,
Set

-----Original Message-----
From: firebird-support@yahoogroups.com [mailto:firebird-support@yahoogroups.com] On Behalf Of Philipp Franzos
Sent: 1. september 2010 10:42
To: firebird-support@yahoogroups.com
Subject: Re: [firebird-support] Firebirs 2.1 : left join right join full join outer join dont' work !

Hi Helen

Thanks for trying to help. Here the exact SQL:

SELECT DISTINCT
PR.SUCHBEGRIFF as Liegenschaft,
AD.VORNAME,
AD.NAME1,
KO.NUMMER,
KO.TYP
FROM
ADRESSE ad,
ADRESSKATEGORIE ak,
ADRESSE_ADRESSKATEGORIE aa,
VERTEILEREINTRAG ve,
PROJEKT pr
left outer join KOMMUNIKATION ko on AD.ID = KO.ADRESSE_ID
WHERE
AK.SUCHBEGRIFF = 'Hauswart' AND
AK.ID = AA.ADRESSKATEGORIE_ID AND
AA.ADRESSE_ID = AD.ID AND
VE.ADRESSE_ID = AD.ID AND
VE.VERTEILER_ID = PR.BETEILIGTEVERTEILER_ID AND
KO.TYP = 'Privat'

Kind Regards
Philipp