Sorting by an arbitrary criterion - Mailing list pgsql-performance

Suppose I have a large table with a small-cardinality CATEGORY column (say, categories 1..5).  I need to sort by an
arbitrary(i.e. user-specified) mapping of CATEGORY, something like this: 

  1 => 'z'
  2 => 'a'
  3 => 'b'
  4 => 'w'
  5 => 'h'

So when I get done, the sort order should be 2,3,5,4,1.

I could create a temporary table with the category-to-key mapping, but is there any way to do this in a single SQL
statement?

Thanks,
Craig