Nick, Brian's right, it's not terribly clear what you're trying to do
with that query:
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) ;
Depending on exactly what you're trying to do, Oracle's lead or lag
commands might be useful, which are discussed a bit in
this old thread.
