Novice SQL Question

I need to the following query:
select distinct event_code, level from logs join stat on (stat.prime is not
null) where order_num = 130680;

Ok, no problem. Does exactly what I want; however, I need to sort this is a
particular way to get the right results. When I try to add the order by
clause, I get an error. Here is the ORDER BY:
ORDER BY event_date DESC, event_time DESC, event_secs DESC

If I put this where I thought it should go as in:
select distinct event_code,level from logs join stat on (stat.prime is not
null) where order_num = 130680 order by event_date,event_time,event_secs;

I get the following error:
ERROR:  For SELECT DISTINCT, ORDER BY expressions must appear in target list

No matter where I put it I get errors. Obviously, I'm a novice. Can somebody
give me any advice? I'm using Postgres 7.2.3 on RedHat.

Thanks...
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