Case statement inside IN

Question

Hi All,
This may be a very simple question but am not able to reason out why it does not work and so posting it for the experts to answer:
I have the below mentioned query:
select username from users where
username IN (
case substr('ABCDEFGHIJKL',8,2)
when 'HI' then (
select distinct(employee.username) from employee,manager
where
employee.userid = manager.userid)
end)
On thrying to execute this statement, I am getting an ORA-01427: single-row subquery returns more than one row error. However, when i run the query:
select username from users where
username IN (
select distinct(employee.username) from employee,manager where employee.userid = manager.userid)
end)
Can anyone let me know the reason behind this error?
Thanks in advance.

Answer 1

I would suspect that this query will retrieve multiple rows
select distinct(employee.username)
                    from employee
                       , manager
                   where employee.userid = manager.userid