you need to post in html mode using the PRE tag to do what you want:
select date_id, n_sessions_day, decode(user_id,null,0,1) member_p,
((select count(distinct user_id) from historical_visits a
where a.date_id = v.date_id)
+ (select count(distinct browser_id) from historical_visits b
where b.date_id = v.date_id and b.user_id is NULL)) n_users
from historical_visits v
group by date_id, n_sessions_day, decode(user_id,null,0,1);
since you are doing a subselect there is no need for a group by at all here.
