MySQL - 在相关子查询中限制聚合数据时嵌套子查询的替代方案

Question

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视图（将其称为 AggregateView），该视图将为每个日期和每个数据列显示一些不同的聚合。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 显示每个的 avg(DailyData1) 1 月 23 日之前的 1 月日期，Data1_20DayAvg 显示相同的结果，但针对表中的前 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结果的数量，我对 20 天平均值很感兴趣。请注意，表中的日期是不规则的，因此我不能使用日期间隔；我需要表中的最后 20 条记录，而不仅仅是过去 20 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。

子查询单独适用于单个硬编码日期：

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

但是尝试将其嵌入作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'

通过搜索，我得到的印象是，您不能将相关子查询用作 from 子句，我认为这就是问题所在。另一个问题是我不确定 MySQL 是否会接受子查询中包含 from 子句的视图定义。有没有办法限制我的聚合选择中的数据而不诉诸子查询，以便解决这两个问题？

Answer 1

不可以，您不能在 FROM 子句中使用关联子查询。但您可以在 ON 条件中使用它们：

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，对于许多日期：

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 

两个查询都假设 Datatable.Date 具有 UNIQUE 约束。