Efficient assignment of percentile/rank in MYSQL - mysql

Question

I have a couple of very large tables (over 400,000 rows) that look like the following:
+---------+--------+---------------+
| ID | M1 | M1_Percentile |
+---------+--------+---------------+
| 3684514 | 3.2997 | NULL |
| 3684515 | 3.0476 | NULL |
| 3684516 | 2.6499 | NULL |
| 3684517 | 0.3585 | NULL |
| 3684518 | 1.6919 | NULL |
| 3684519 | 2.8515 | NULL |
| 3684520 | 4.0728 | NULL |
| 3684521 | 4.0224 | NULL |
| 3684522 | 5.8207 | NULL |
| 3684523 | 6.8291 | NULL |
+---------+--------+---------------+...about 400,000 more
I need to assign each row in the M1_Percentile column a value that represents "the percent of rows with M1 values equal or lower to the current row's M1 value"
In other words, I need:
I implemented this sucessfully, but it is FAR FAR too slow. If anyone could create a more efficient version of the following code, I would really appreciate it!
UPDATE myTable AS X JOIN (
SELECT
s1.ID, COUNT(s2.ID)/ (SELECT COUNT(*) FROM myTable) * 100 AS percentile
FROM
myTable s1 JOIN myTable s2 on (s2.M1 <= s1.M1)
GROUP BY s1.ID
ORDER BY s1.ID) AS Z
ON (X.ID = Z.ID)
SET X.M1_Percentile = Z.percentile;
This is the (correct but slow) result from the above query if the number of rows is limited to the ones you see (10 rows):
+---------+--------+---------------+
| ID | M1 | M1_Percentile |
+---------+--------+---------------+
| 3684514 | 3.2997 | 60 |
| 3684515 | 3.0476 | 50 |
| 3684516 | 2.6499 | 30 |
| 3684517 | 0.3585 | 10 |
| 3684518 | 1.6919 | 20 |
| 3684519 | 2.8515 | 40 |
| 3684520 | 4.0728 | 80 |
| 3684521 | 4.0224 | 70 |
| 3684522 | 5.8207 | 90 |
| 3684523 | 6.8291 | 100 |
+---------+--------+---------------+
Producing the same results for the entire 400,000 rows takes magnitudes longer.

Answer 1

I cannot test this, but you could try something like:
update table t
set mi_percentile = (
select count(*)
from table t1
where M1 < t.M1 / (
select count(*)
from table));
UPDATE:
update test t
set m1_pc = (
(select count(*) from test t1 where t1.M1 < t.M1) * 100 /
( select count(*) from test));
This works in Oracle (the only database I have available). I do remember getting that error in MySQL. It is very annoying.

Answer 2

Fair warning: mysql isn't my native environment. However, after a little research, I think the following query should be workable:
UPDATE myTable AS X
JOIN (
SELECT X.ID, (
SELECT COUNT(*)
FROM myTable X1
WHERE (X.M1, X.id) >= (X1.M1, X1.id) as Rank)
FROM myTable as X
) AS RowRank
ON (X.ID = RowRank.ID)
CROSS JOIN (
SELECT COUNT(*) as TotalCount
FROM myTable
) AS TotalCount
SET X.M1_Percentile = RowRank.Rank / TotalCount.TotalCount;