Multi value parameter in SQL command - CR2013

Im using CR2013 to query an oracle 11 database.

I want to use a multi value parameter in my sql query (rather than using the parameter in the select expert).
 So far I have:

1. created a parameter in the 'Command Parameter' dialog box called p_student
2. checked the 'Allows multiple values' checkbox in the 'Command Parameter' dialog box
3. added the parameter into the query using syntax:
       where student.studentname in ({?p_student})

4. added the following values into the p_student parameter:
      George
      Paul
      Ringo
      John

This seems to work fine, but now I want to add an additional parameter option called 'All students':

All students
      George
      Paul
      Ringo
      John

Which when selected would pass the following sql into the query:

      Where Student.studentname like ‘%’

So would return all students

The logic would be

Case when
{?p_student} = ‘All Students’ then
where Student.studentname like ‘%’
else
where student.studentname in ({?p_student})
end

Any help with the syntax is appreciated.