SELECT AVG(V.TOTAL_VALUE) AS SA_MEDIAN
FROM 
(
  SELECT
    con.TOTAL_VALUE
  , ROW_NUMBER() OVER (ORDER BY con.TOTAL_VALUE) AS ren
  , COUNT(*) OVER () AS cnt
  FROM CONVENIENCE con
) V
WHERE V.cnt/2.0 BETWEEN V.ren - 1 AND V.ren