如何在MySQL中按组累计总和

Question

我想知道每个月有多少在职学生出于统计目的。

当前数据类似于此伪表：

start_date  quit_date   dummyName
-----------------------------------------
2015-09-12  2015-12-12  foo
2015-10-12  2015-12-12  bar
2015-10-13  2015-12-12  bob
2015-10-13  2015-12-12  rich
2015-12-13  2015-12-31  eve

结果将按月分组。 GROUP BY和SUM不起作用，它必须像这样累积：

month       count
-----------------
2015-08     0    # no student added
2015-09     1    # just one student was added in this month
2015-10     4    # three more students added in october
2015-11     4    # no student added
2015-12     5    # one more student added
2016-01     0    # all students quits in december, so january there is no students

我该如何查询？

Answer 1

对于像这样的少数几行，您可以创建一个表，其中包含要报告的日期，并使用相关的子查询：

select date_format(eom, '%Y-%m') as yyyymm,
       (select count(*) 
        from dummy du
        where du.start_date <= d.eom and
              du.quit_date > d.eom
       ) NumStudents
from (select date('2015-08-31') as eom union all
      select date('2015-09-30') as eom union all
      select date('2015-10-31') as eom union all
      select date('2015-11-30') as eom union all
      select date('2015-12-31') as eom union all
      select date('2016-01-31') as eom
     ) d;