Fetch cumulative sum from MySQL table

I have a table containing donations, and I am now creating a page to view statistics. I would like to fetch monthly data from the database with gross and cumulative gross.

mysql> describe donations;
+------------------+------------------+------+-----+---------+----------------+
| Field            | Type             | Null | Key | Default | Extra          |
+------------------+------------------+------+-----+---------+----------------+
| id               | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| transaction_id   | varchar(64)      | NO   | UNI |         |                |
| donor_email      | varchar(255)     | NO   |     |         |                |
| net              | double           | NO   |     | 0       |                |
| gross            | double           | NO   |     | NULL    |                |
| original_request | text             | NO   |     | NULL    |                |
| time             | datetime         | NO   |     | NULL    |                |
| claimed          | tinyint(4)       | NO   |     | NULL    |                |
+------------------+------------------+------+-----+---------+----------------+

Here's what I've tried:

SET @cgross = 0;
SELECT YEAR(`time`), MONTH(`time`), SUM(`gross`), (@cgross := @cgross + SUM(`gross`)) AS `cumulative_gross` FROM `donations` GROUP BY YEAR(`time`), MONTH(`time`);

The result is:

+--------------+---------------+--------------+------------------+
| YEAR(`time`) | MONTH(`time`) | SUM(`gross`) | cumulative_gross |
+--------------+---------------+--------------+------------------+
|         2013 |             1 |          257 |              257 |
|         2013 |             2 |          140 |              140 |
|         2013 |             3 |          311 |              311 |
|         2013 |             4 |          279 |              279 |
+--------------+---------------+--------------+------------------+

Which is wrong. The desired result would be:

+--------------+---------------+--------------+------------------+
| YEAR(`time`) | MONTH(`time`) | SUM(`gross`) | cumulative_gross |
+--------------+---------------+--------------+------------------+
|         2013 |             1 |          257 |              257 |
|         2013 |             2 |          140 |              397 |
|         2013 |             3 |          311 |              708 |
|         2013 |             4 |          279 |              987 |
+--------------+---------------+--------------+------------------+

I tried this without SUM, and it did work as expected.

SET @cgross = 0;
SELECT YEAR(`time`), MONTH(`time`), SUM(`gross`), (@cgross := @cgross + 10) AS `cumulative_gross` FROM `donations` GROUP BY YEAR(`time`), MONTH(`time`);

+--------------+---------------+--------------+------------------+
| YEAR(`time`) | MONTH(`time`) | SUM(`gross`) | cumulative_gross |
+--------------+---------------+--------------+------------------+
|         2013 |             1 |          257 |               10 |
|         2013 |             2 |          140 |               20 |
|         2013 |             3 |          311 |               30 |
|         2013 |             4 |          279 |               40 |
+--------------+---------------+--------------+------------------+

Why doesn't it work with SUM? Any ideas how I could fix it?

Thanks, Lassi

A subquery without variables will do it just as easily, and quite a bit more portably;

SELECT YEAR(`time`), 
       MONTH(`time`), 
       SUM(gross),
       (SELECT SUM(gross) 
        FROM donations 
        WHERE `time`<=MAX(a.`time`)) cumulative_gross
FROM donations a GROUP BY YEAR(`time`), MONTH(`time`);

An SQLfiddle to test with.