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create table test (id char(5) not null, datum date not null);
commit;
alter table test add constraint pk_test primary key (id,datum);
commit;
insert into test (id, datum) values ('08001',cast('01.01.2008' as date));
insert into test (id, datum) values ('08001',cast('01.02.2008' as date));
insert into test (id, datum) values ('08001',cast('01.03.2008' as date));
insert into test (id, datum) values ('08002',cast('01.02.2008' as date));
insert into test (id, datum) values ('08002',cast('01.03.2008' as date));
insert into test (id, datum) values ('08003',cast('01.03.2008' as date));
commit;

select distinct datum from test order by datum;

raised exception: "field id not found".

Probably this is internal exception in dbExpress that is not shown to the user. You can do the following to resolve the problem:
Open Tools | Options; select Debugger Options \ CodeGear Debuggers \ Language Exceptions category, and uncheck 'Notify on language exceptions' checkbox.

Thank you.
Your advice works.
Regards, Martin Zwollo