Table is users, columns are user_id, firstname, lastname, area.
The other table is user_sessions, and columns are user_id, logon, logoff.
To see who is logged on I use
select u.FIRSTNAME, u.LASTNAME, u.PHONE_ID, us.LOGON
from USERS u
join USER_sessions us on u.USER_ID=us.user_id
where cast (us.LOGON as date) = date 'now'
and us.LOGOFF is null
order by u.FIRSTNAME
The result is correct, but sometimes I get duplicated entries.
For example, same firstname and lastname, but different logon. I'd like to only see the latest logon.
It looks like field LOGON is kind of a timestamp type field; your question is not really precise, there.
So, I think, you want the maximum value of LOGON. For that, there is an aggregate function MAX() which has to be used with a GROUP BY.
select u.FIRSTNAME, u.LASTNAME, u.PHONE_ID, MAX(us.LOGON)
from USERS u
join USER_sessions us
on u.USER_ID = us.user_id
where cast (us.LOGON as date) = date 'now'
and us.LOGOFF is null
group by u.FIRSTNAME, u.LASTNAME, u.PHONE_ID
order by u.FIRSTNAME
You'll get the latest LOGON for every occurence of u.FIRSTNAME, u.LASTNAME, u.PHONE_ID.
Note, that you can use current_date context variable instead of date 'now' cast.