sql question

I have a sql problem which I can't solve. The following table is defined

create table AdressGroup
(
   AdrGroup_Id                     INTEGER       NOT NULL
         DEFAULT NEXTVAL('adrverw_id_seq'),
   Zeit                            DATETIME      NOT NULL,
   Group_Id                        INTEGER       NOT NULL,
   Adress_Id                       INTEGER       NOT NULL
);

The table assigns adresses to adressgroups.

I'd like to select the adresses which belongs to one specific adressGroup and to no other group. If an adress has more
thanone entry in the AdressGroup 
table it should not be in the projection.

I tried the following:

SELECT * FROM adressGroup
 WHERE Group_Id = 6
   AND EXISTS( SELECT AdrGroup_Id FROM adressGroup ag_alias
                WHERE adressGroup.AdrGroup_Id = ag_alias.AdrGroup_Id
                GROUP BY AdrGroup_Id HAVING COUNT(*) == 1 );

When I do this I get the following error:
  ERROR:  rewrite: aggregate column of view must be at rigth side in qual
  ERROR:  rewrite: aggregate column of view must be at rigth side in qual


Does anybody knows the solution? Thanks

--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Herbert Liechti                            E-Mail: Herbert.Liechti@thinx.ch
ThinX networked business services               Stahlrain 10, CH-5200 Brugg
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