ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

Hi all!
can anybody say me what's wrong with this query. I just try to take
unique values from table column and print them in random order

select distinct num from (select 1 as num union select 2 as num union
select 1 as num union select 3) t order by random();
ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list

thank you