Decode and group by Query

Question

The following query throws up 'Not a group by Expression' Error:
select decode(visited_last_date,sysdate,'Present','null') ,forum_name
from disc_forum f, disc_emp_pref p
where f.forum_category='TSP'
and f.forum_short_name=p.forum_short_name group by f.forum_short_name
How can i get this working? I need to retain the group-by clause.

Answer 1

Thanx Detlev,
The query works with the group fn. But the actual query that i'm working on is:
select DECODE(visited_last_date,sysdate,'<img src="/images/on.gif">','null') img, ''&#0124; &#0124;f.forum_name&#0124; &#0124;'' forum_name, count(message_id) Posts, to_char(max(m.posted_date),'DD-MON-YYYY') last_updated from disc_message m, disc_thread t,disc_forum f,disc_emp_pref p where f.forum_category=nvl(:category,f.forum_category) and f.forum_short_name=t.forum_short_name(+) and t.thread_id=m.thread_id(+) group by cube(f.forum_name,f.forum_short_name,f.forum_description,p.visited_last_date)
I'm using this query to build a report in Oracle Portal. This query is working only that i'm getting duplicate rows. If i don't include the visited_last_date in the group_by clause then it flags a 'Not a group-by expr' error. How do i avoid that