Select by specified order

Question

Hi,
In the table, (testtable). That are contain pair values var -> num : a - > 1, b -> 2, c -> 3 .......... z ->26 .
I want to select var from testtable by specified order by
num in (1,3,5,2) the var order to be a,c,e,b
num in (5,4,3,2,1) the var order to be e,d,c,b,a
But the result is not!
Is there a way to select vars by specified order by num field?
Thank you!
Francis SZE
SQL> desc testtable;
Name                                      Null?    Type
VAR                                       NOT NULL VARCHAR2(20)
NUM                                       NOT NULL NUMBER
SQL> select var from testtable where num in (1,3,4);
VAR
a
c
d
SQL> select var from testtable where num in (1,3,5,2);
VAR
a
b
c
e
SQL> select var from testtable where num in (5,4,3,2,1);
VAR
a
b
c
d
e

Answer 1

SQL> with sample_table as (
  2                        select level num,chr(ascii('a') + level - 1) var from dual connect by level <= 26
  3                       )
  4  select  var
  5    from  sample_table
  6    where num in (1,3,5,2)
  7    order by case num
  8               when 3 then 2
  9               when 5 then 3
10               when 2 then 4
11               else num
12             end
13  /
V
a
c
e
b
SQL> SY.