我想逐月计算 Jmeter 值。我的数据如下：

| Date       | Energy_produced |
--------------------------------
| 2011-01-01 | 12              |  
| 2017-01-02 | 23              |  
| 2017-01-03 | 25              |  
...
| 2011-02-01 | 11              |  
| 2017-02-02 | 12              |  
| 2017-02-03 | 17              |

group by将得出每个月的“energy\u Producted”值的总和，但我希望将总和相加得到：

| Month   | Energy_produced_added_up |
--------------------------------------
| 2011-01 | 60                       |  
| 2017-02 | 100                      |

我读了很多文章，明白我可以使用一个变量：

set @result := 0;
SELECT 
    DATE_FORMAT(t.Datum_Maand, '%m') AS measurement_month, 
    DATE_FORMAT(t.Datum_Maand, '%Y') AS measurement_year,
    (@result := @result + SUM(t.Geg_Maand)) AS SUMME
FROM tgeg_maand AS t
GROUP BY measurement_year, measurement_month
ORDER BY measurement_year, measurement_month

使用group by时，值不会相加。上面的语句显示的结果如下：

SELECT 
    DATE_FORMAT(t.Datum_Maand, '%m') AS measurement_month, 
    DATE_FORMAT(t.Datum_Maand, '%Y') AS measurement_year,
    SUM(t.Geg_Maand) AS SUMME
FROM tgeg_maand AS t
GROUP BY measurement_year, measurement_month
ORDER BY measurement_year, measurement_month

我做错什么了？
谢谢