Retrieve a percentile rankings in MySQL.

-- ---------------------------------------------------------------------------
-- Percentiles
--
-- from the Artful Common Queries page
-- http://www.artfulsoftware.com/infotree/qrytip.php?id=68
-- ---------------------------------------------------------------------------

-- In the Sakila table film, retrieve a top-down percentile ranking of
-- film lengths:

SELECT
  a.film_id ,
  ROUND( 100.0 * ( SELECT COUNT(*) FROM film AS b WHERE b.length <= a.length ) / total.cnt, 1 )
  AS percentile
FROM film a
CROSS JOIN (
  SELECT COUNT(*) AS cnt
  FROM film
) AS total
ORDER BY percentile DESC;

-- If there are NULLs, filter them out before computing percentiles.

On his blog, Roland Bouman shows a much faster query
(http://rpbouman.blogspot.com/2008/07/calculating-percentiles-with-mysql.html);

-- here is a version retrieving the first film at or above the 90th percentile:

SELECT
  SUM(g1.r) sr,
  g2.length l,
  SUM(g1.r)/(SELECT COUNT(*) FROM film) p
FROM (
  SELECT COUNT(*) r, length
  FROM film
  GROUP BY length
) g1
JOIN (
  SELECT COUNT(*) r, length
  FROM film
  GROUP BY length
) g2 ON g1.length < g2.length
GROUP BY g2.length
HAVING p >= 0.9
ORDER BY p LIMIT 1