按月分组并显示剩余的总天数

Question

我想列出按月分组以及在下一栏中完成课程的剩余天数。课程有10天。

示例数据

ID Name Date 
1 Sandy 2015-05-06 
2 Candy 2015-05-06 
3 Sandy 2015-05-28 
4 Candy 2015-05-29
5 Candy 2015-06-01

首选输出

| Name | Month | Attended | Remaining|
| Sandy| May   |   2      |     8    |
| Candy| May   |   2      |     8    |
| Candy| June  |   1      |     7    |

如果我使用GROUP BY DATE_FORMAT（日期，'％Y％m'），名称并尝试进行计算则不起作用。

Answer 1

您需要两种不同的聚合：

1. 指定用户当月参加的天数。
2. 在指定用户的当月之前的所有月份中参加的天数。

这有点繁琐，所以现在是时候做一些测试驱动的查询设计（TDQD）。

问题中的表格是匿名的 - 这是一种常见且令人不快的情况。因此，该表从此为CourseAttendance，数据中显示了三列（ID，名称，日期）。

用户在特定月份参加的天数

假设表达式DATE_FORMAT(date, '%Y-%m')在语法上有效，并且Date和Month作为列名称都不会导致问题，那么：

SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
       Name,
       COUNT(*) AS NumDays
  FROM CourseAttendance
 GROUP BY Month, Name

这应该产生：

Month     Name      NumDays
2015-05   Sandy     2
2015-05   Candy     2
2015-06   Candy     1

用户在特定月份（包括特定月份）参与的天数

这次，汇总必须超过所有日期小于或等于转换月份值：

SELECT D.Month, D.Name, SUM(C.NumDays) AS TotDays
  FROM (SELECT DISTINCT DATE_FORMAT(Date, '%Y-%m') AS Month, Name
          FROM CourseAttendance
       ) AS D
  JOIN (SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
               Name,
               COUNT(*) AS NumDays
          FROM CourseAttendance
         GROUP BY Month, Name
       ) AS C
    ON C.Month <= D.Month AND C.Name = D.Name
 GROUP BY D.Month, D.Name

这应该给出输出：

Month     Name      NumDays
2015-05   Sandy     2
2015-05   Candy     2
2015-06   Candy     3

汇总最终结果

前两个结果表需要在Month和Name上连接，以产生结果：

SELECT A.Name, A.Month, A.NumDays AS Attended, (10 - B.TotDays) AS Remaining
  FROM (SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
               Name,
               COUNT(*) AS NumDays
          FROM CourseAttendance
         GROUP BY Month, Name
       ) AS A
  JOIN (SELECT D.Month, D.Name, SUM(C.NumDays) AS TotDays
          FROM (SELECT DISTINCT DATE_FORMAT(Date, '%Y-%m') AS Month, Name
                  FROM CourseAttendance
               ) AS D
          JOIN (SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
                       Name,
                       COUNT(*) AS NumDays
                  FROM CourseAttendance
                 GROUP BY Month, Name
               ) AS C
            ON C.Month <= D.Month AND C.Name = D.Name
         GROUP BY D.Month, D.Name
       ) AS B
    ON A.Month = B.Month AND A.Name = B.Name
 ORDER BY A.Name, A.Month

这应该输出如下：

Name      Month      Attended     Remaining
Candy     2015-05    2            8
Candy     2015-06    1            7
Sandy     2015-05    2            8

如果需要，您可以将月份值伪装成月份名称。如果您想在月份内使用月份和名称等，也可以获取排序顺序