SQL & PL/SQL

I have the following query:

select *
  from (select employee_id, email,
               case
                  when dbms_random.value () < 0.5
                     then 'S'
               end as status
          from hr.employees)
 where status is not null

The inner query does what I expect, namely return all employee records and about 50% of them have the 'S' status. Then I try to use the outer query to filter only employees with status 'S'. What I get, in this case, is about half the time no rows returned and about half the time all rows returned (including those with null status).

Any ideas?

Edited by: user6570265 on Jun 22, 2009 3:19 PM