updating duplicate rows

i wa开发者_如何学编程nt to change 'aa' to 'bb' and 'bb' to 'aa'

col1 col2
---- ----
1    aa
2    bb
3    aa
4    aa
5    bb

is it possible with single update statement? i know it is possible with some temp table or variable

For example:

UPDATE your_table_name SET 
  col2=CASE WHEN col2='aa' THEN 'bb'
            WHEN col2='bb' THEN 'aa'
            ELSE col2
       END