SQL & PL/SQL

Hi Expert,

I want to do concatenate string with number , example I have 6 i want to add 0 before 6 result 06.

see query

select 
      case 
      when (floor(((o-i)*24*60*60)/3600)) > 9 then  
      floor(((o-i)*24*60*60)/3600)
      else           
      0 || to_char(floor(((o-i)*24*60*60)/3600))
      End 
      Wh
from T

showing error ORA-00932 inconsistent datatypes:

please guide.