can I use a if clause within a where clause?

Question

where 
     if prog.levelnumber = 2 then
         i.level = 2 ---> select only the persons with level = 2 
     if prog.numnivel=1 then
         i.level = 1 ---> select only the persons with level = 1

Answer 1

This looks like SQL to me, so I'm answering it in that way...

Use a case statement:

select fields
  from tablename
 where i.level = case prog.levelnumber
                   when 2 then 2
                   else
                     case prog.numnivel
                       when 1 then 1
                     end
                 end