ORDEN SQL condicional POR ASC/DESC para columnas alfa

Una opción

;WITH cQuery AS
(
   SELECT
       *,
       ROW_NUMBER() OVER (ORDER BY SortColumn) AS RowNum
   FROM
       MyTable
)
SELECT
   *
FROM
   cQuery
ORDER BY
   RowNum * @Direction --1 = ASC or -1 = DESC

O CASE que en mi humilde opinión es un poco más feo

ORDER BY
  CASE WHEN 'ASC' THEN SortColumn ELSE '' END ASC,
  CASE WHEN 'DESC' THEN SortColumn ELSE '' END DESC