select distinct w/order by

I know this is an old topic, but it's not easy to find a way around it, so
when we migrate SQL from other database to PostgreSQL, it causes a huge
headache. Here's an extremely simple example -

The original simple SQL -
select distinct atcode from TMP order by torder;

(it'll error out in PostgreSQL, although SQL92 extension may allow it;
there's time you just can't do "select distinct atcode,torder from TMP order
by torder"!!)

My desire result -
HGB
HCT
WBC
RBC
MCV
MCH
MCHC
RDW
RDWSD
PLT
DIFF | TYPE
SEGS
LYMPHS
MONOS
EOS
BASOS

I tried to rewrite the above simple query in PostgreSQL as - select distinct
atcode from (select atcode,torder from TMP order by torder) t;

But the return results are not what I want - BASOS DIFF | TYPE EOS HCT
HGB LYMPHS MCH MCHC MCV MONOS PLT RBC RDW RDWSD SEGS WBC

Can anybody provide a real/general solution to the above practical problem?
(Tom?) This causes postgreSQL users too much time and headache.

Thanks.
johnl