Yes I know before hand the user_id's but the problem is that I am not searching in only one table for the id's and that is where I get stuck with the decode function, I haven't use it before so I don't know how the syntax for searching in several tables for several data is like.
I've been looking at the decode syntax and definition and I think this is not the function that I need. I think that decode returns only one value and I need more...

Can you please explain me how could I use this in my query, I've been trying to make it work with decode and count with no luck.

Thanks for your reply...

I want to get some information from a database in wich there are events of certain dates that I want to count and group by the date for several users.

Until now I have made the query for counting and grouping one user.

SELECT
emp.first_name || ' ' || emp.last_name AS Employee_Name,
to_char(e.start _date_time,'MM/DD/YYYY') as DATE,
count(e.start_d ate_time) COUNT
FROM
event e,
...