Oracle: sorting by an aliased column

I recently added an ad hoc column to a heavily used query with paging and sorting. To get the sorting to work on the new aliased column it was necessary to pull the "row_number() over" clause in to an outer query. I just wanted to document how this was done for future reference.

 
  SELECT *
    FROM (SELECT my_table.*,
                 ROW_NUMBER ()
                 OVER (
                    ORDER BY
                       UPPER (
                          DECODE (
                             pSortDir,
                             'asc', DECODE (pSortOrder,
                                       'HasDemographicData', has_demographic_data
                                       pc.legal_name))),
                       UPPER (
                          DECODE (
                             pSortDir,
                             'desc', DECODE (pSortOrder,
                                        'HasDemographicData', has_demographic_data
                                        pc.legal_name))) DESC)
                    rn
            FROM (SELECT /*+ first_rows(20) */
                         pc.dot_number,
                         ca.carrier_operation,
                         pc.dba_name,
                         ca.phy_street,
                         pc.isp_annual_screenings,
                         CASE
                            WHEN (pc.discovered_psp_via IS NULL) THEN 'No'
                            ELSE 'Yes'
                         END
                            AS has_demographic_data
                    FROM psp_customer pc
                         LEFT OUTER JOIN carrier ca
                            ON pc.dot_number = TO_CHAR (ca.dot_number)
                   WHERE pDot_number IS NULL OR pc.dot_number LIKE '%' || pDot_number || '%')
   WHERE rn BETWEEN vStartRow AND vEndRow
ORDER BY rn;