This actually answers the question, although I would imagine the ORDER BY handling will look pretty messy (CASE statement perhaps?). And I don't know what it w

This actually answers the question, although I would imagine the ORDER BY handling will look pretty messy (CASE statement perhaps?). And I don't know what it would look like if the requirements changed to allow ordering by multiple columns with different possible sort directions... that might get back into dynamic SQL using a RETURN QUERY EXECUTE type of thing, which is basically using a query builder in your query language.