MYSQL SUM GROUP BY [英] MYSQL SUM GROUP BY

问题描述

我正在一所高中的分级制度。

I'm working on a high school grading system.

在我的学校，成绩可以通过再加工的问题而改变，我保存这些变化的日期。

At my school, grades can be changed by reworking problems and I store these changes with dates.

我有一个正确返回平均值的函数，因为最近的等级被标记为值为'1'的当前字段。我希望使函数能够返回过去某个日期的最新成绩。我正在绘制他们的平均值如何随时间变化的图表。

I have a function that properly returns averages because the most recent grade is flagged with a "current" field with a value of '1'. I'd like to make the function capable of returning the most recent grade with respect to a date in the past. I'm making a graph of how their average has changed over time.

我想要做的是这样的：

select sum(grades.points) 
  from grades 
 where date < 'thedate' 
order by date DESC 
group by assignmentID

我不能t用和和分组。它错误...

I can't use sum and group by. It errors...

我能想到的最好的做法是做一个子选择。 ？任何其他的想法。

The best I can think of is to do a sub-select. Any other thoughts?

推荐答案

GROUP BY有来ORDER BY之前：

GROUP BY has to come before ORDER BY:

  SELECT SUM(g.points) 
    FROM GRADES g
   WHERE g.date < 'thedate' 
GROUP BY g.assignmentid
ORDER BY g.date DESC 

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