MySQL Percentile aftermath: Calculating all quantiles

Are you getting fed up yet with my posts on calculating percentiles? Well, I'm sorry but I'm not quite finished.

Here's a simple, fast method to calculate the specified number of quantiles:

-- set the number of quantiles, for exmple:
--   quartiles: 4
--   deciles: 10
--   percentiles: 100

SET @quantiles:=4;  -- select quartiles

-- calculate all quantiles
-- 
--
SELECT     amount                     AS metric
,          @n DIV (@c DIV @quantiles) AS quantile
,          @n                         AS N
FROM       sakila.payment
CROSS JOIN (
            SELECT @n:=0                           -- rownumber
            ,      @c:=COUNT(*)                    -- need this to calculate quantile partitions
            FROM   sakila.payment
           ) c
WHERE      NOT (                                   -- modulo zero (=false), we are at the quantile
               (@n:=@n+1) % (@c DIV @quantiles)    -- rownumber equal to the quantile partition? 
           ) 
ORDER BY   amount;                                 -- need ordered partitions

You can find this snippet on MySQL Forge.