查找前 30 个结果的平均值

Question

我有一个查询要输出前 30 个结果

select top 30 * from [dbo].[Stats]
        where Rack_Code = 'ABC'
        order by Date_of_Record desc

并查询以获取所有时间的平均值

SELECT AVG(Total_MB - (East_MB + West_MB)) AS Other,
                    AVG(East_MB) AS East,
                    AVG(West_MB) AS West
                    FROM [dbo].[Stats]
                    WHERE Rack_Code = 'ABC'

给出正确的输出

但是当我尝试获取前 30 个条目的平均值时，SQL 查询似乎忽略了 Top 30

SELECT TOP 30 AVG(Total_MB - (East_MB + West_MB)) AS Other,
                    AVG(East_MB) AS East,
                    AVG(West_MB) AS West
                    FROM [dbo].[Stats]
                    WHERE Rack_Code = 'ABC'

并给出不正确的输出

请指教

Answer 1

您几乎拥有它，您所要做的就是将您的 TOP 30 放在子查询或 CTE 中，而不是您的 AVG

SELECT AVG(Total_MB - ( East_MB + West_MB )) AS Other
   ,AVG(East_MB) AS East
   ,AVG(West_MB) AS West
FROM (
       SELECT TOP 30 *
        FROM [dbo].[stats]
        WHERE Rack_Code = 'ABC'
        ORDER BY Date_of_Record DESC
     ) a

请注意，只有在有TOP 子句时才能在子查询中使用ORDER BY。

CTE 解决方案，

;WITH   base
          AS (
               SELECT TOP 30 *
                FROM [dbo].[stats]
                WHERE Rack_Code = 'ABC'
                ORDER BY Date_of_Record DESC
             )
    SELECT AVG(Total_MB - ( East_MB + West_MB )) AS Other
           ,AVG(East_MB) AS East
           ,AVG(West_MB) AS West
        FROM base

Documentataion 在 CTE 上。请注意，SQL-Server 将 CTE 视为视图。这可能会引入一些开销，如果嵌套足够深，则查询优化器将无法读取统计信息并可能导致性能大幅下降。