Median in MySQL

quantile.sql

SET @rownum := -1;
SET @quantile := 0.5;

SELECT
   AVG(t.field)
FROM
(
   SELECT
      @rownum := @rownum + 1 AS rownum,
      table.field AS field
   FROM
      table
   ORDER BY table.field
) AS t
WHERE
   t.rownum IN (
      CEIL(@rownum * @quantile),
      FLOOR(@rownum * @quantile)
   )
;