我有一个看起来像这样的表:

DataTable
+------------+------------+------------+
| Date       | DailyData1 | DailyData2 |
+------------+------------+------------+
| 2012-01-23 |     146.30 |     212.45 |
| 2012-01-20 |     554.62 |     539.11 |
| 2012-01-19 |     710.69 |     536.35 |
+------------+------------+------------+

我正在尝试创建一个 View (将其称为 AggregateView)，该 View 将为每个日期和每个数据列显示几个不同的聚合。例如，select * from AggregateView where Date = '2012-01-23' 可能会给出:

+------------+--------------+----------------+--------------+----------------+
| Date       | Data1_MTDAvg | Data1_20DayAvg | Data2_MTDAvg | Data2_20DayAvg |
+------------+--------------+----------------+--------------+----------------+
| 2012-01-23 |       697.71 |         566.34 |       601.37 |         192.13 |
+------------+--------------+----------------+--------------+----------------+

其中 Data1_MTDAvg 显示 1 月 23 日之前的每个日期的 avg(DailyData1)，Data1_20DayAvg 显示相同但之前的日期表中有 20 个日期。我不是 SQL 忍者，但我认为最好的方法是通过子查询。 MTD 平均值很简单:

select t1.Date, (select avg(t2.DailyData1) 
                 from DataTable t2 
                 where t2.Date <= t1.Date 
                   and month(t2.Date) = month(t1.Date) 
                   and year(t2.Date) = year(t1.Date)) Data1_MTDAvg 
from DataTable t1;

但由于需要限制返回的结果数量，我对 20 天的平均值感到困惑。请注意，表格中的日期是不规则的，所以我不能使用日期间隔；我需要表中的最后二十条记录，而不仅仅是过去二十天的所有记录。我找到的唯一解决方案是使用嵌套子查询首先限制所选记录，然后取平均值。

子查询单独用于单独的硬编码日期:

select avg(t2.DailyData1) Data1_20DayAvg 
from (select DailyData1 
      from DataTable 
      where Date <= '2012-01-23' 
      order by Date desc 
      limit 0,20) t2;

但是尝试将其嵌入为更大查询的一部分会失败:

select t1.Date, (select avg(t2.DailyData1) Data1_20DayAvg 
                 from (select DailyData1 
                       from DataTable 
                       where Date <= t1.Date 
                       order by Date desc 
                       limit 0,20) t2) 
from DataTable t1;
ERROR 1054 (42S22): Unknown column 't1.Date' in 'where clause'

通过四处搜索，我得到的印象是您不能将相关子查询用作 from 子句的一部分，我认为这就是问题所在。另一个问题是我不确定 MySQL 是否会接受在子查询中包含 from 子句的 View 定义。有没有一种方法可以在不求助于子查询的情况下限制我的聚合选择中的数据，以解决这两个问题？

最佳答案

不，您不能在 FROM 子句中使用相关的子查询。但是您可以在 ON 条件下使用它们:

SELECT AVG(d.DailyData1) Data1_20DayAvg 
       --- other aggregate stuff on d (Datatable)
FROM 
      ( SELECT '2012-01-23' AS DateChecked
      ) AS dd
  JOIN
      DataTable AS d
    ON
      d.Date <= dd.DateChecked
    AND
      d.Date >= COALESCE( 
      ( SELECT DailyData1 
        FROM DataTable AS last20 
        WHERE Date <= dd.DateChecked 
          AND (other conditions for last20)
        ORDER BY Date DESC 
        LIMIT 1 OFFSET 19
      ), '1001-01-01'   )
WHERE (other conditions for d Datatable)

类似的，对于很多日期:

SELECT dd.DateChecked 
     , AVG(d.DailyData1) Data1_20DayAvg 
       --- other aggregate stuff on d (Datatable)
FROM 
      ( SELECT DISTINCT Date AS DateChecked
        FROM DataTable 
      ) AS dd
  JOIN
      DataTable AS d
    ON
      d.Date <= dd.DateChecked
    AND
      d.Date >= COALESCE( 
      ( SELECT DailyData1 
        FROM DataTable AS last20
        WHERE Date <= dd.DateChecked 
          AND (other conditions for last20)
        ORDER BY Date DESC 
        LIMIT 1 OFFSET 19
      ), '1001-01-01'   )
WHERE (other conditions for d Datatable)
GROUP BY 
      dd.DateChecked 

这两个查询都假定 Datatable.Date 具有 UNIQUE 约束。

关于MySQL - 在相关子查询中限制聚合数据时嵌套子查询的替代方法，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9068777/