SQL Server-分組依據，平均值和百分數

Question

我在SQL Server中有一個FormSummaries表，其中包含示例數據的以下相關列：

FormName | CompletionTime
Form1    | 70
Form1    | 20
Form1    | 30
Form1    | 40
Form1    | 80
Form1    | 60
Form1    | 90
Form1    | 10
Form2    | 30
Form2    | 40
Form2    | 80
Form2    | 90
Form2    | 40
Form2    | 1000
Form2    | 120
Form2    | 70

我需要做的是：

1）按表單名稱和該表單的平均完成時間分組數據，這很容易：

SELECT 
    FormName, AVG(CompletionTime) 
FROM 
    FormSummaries 
WHERE 
    CompletionTime  is not null
GROUP BY
    FormName

2）獲取每種表格類型的最高25％/最低25％的完成時間的平均值（即，完成表格所需的平均最快和最慢25％的時間）。 理想情況下，這將在一個查詢中，即

FormName | Bottom25%AverageCompletionTime | Top25%AverageCompletionTime
Form1    | 85                             | 15
Form2    | 560                            | 35

我生活在現實世界中，意識到可能不可能，因此分別查詢頂部和底部會很好，例如

FormName | Bottom25%AverageCompletionTime
Form1    | 85                            
Form2    | 560                           

FormName | Top25%AverageCompletionTime
Form1    | 15
Form2   | 35

我已經看過Ntile和Over的Partition by，但是我似乎無法獲得任何東西來產生想要的結果（盡管那很可能是因為我沒有正確實現這些！）。

有人可以幫忙嗎？

非常感謝。

Answer 1

NTILE居塊的結果，所以你有興趣在宿舍，所以使用NTILE（4）分成4組，分區在表格名稱。 為此，請嘗試2個查詢

-- top 25%
SELECT  formname, AVG(CompletionTime) 
FROM
(SELECT 
    FormName,completiontime, NTILE(4) over (partition by FormName order by completiontime) as QuartPercentile
FROM 
    FormSummaries
WHERE CompletionTime IS NOT NULL )
    x
WHERE  QuartPercentile = 1
GROUP BY formname

-- bottom 25%
SELECT  formname, AVG(CompletionTime) 
FROM
(SELECT 
    FormName,completiontime, NTILE(4) over (partition by FormName order by completiontime) as QuartPercentile
FROM 
    FormSummaries 
WHERE CompletionTime IS NOT NULL)
    x
WHERE  QuartPercentile = 4
GROUP BY formname

或一次查詢

SELECT  formname,AVG( case when QuartPercentile = 4 then CompletionTime else null end)   as [Bottom25%AverageCompletionTime]
, AVG( case when QuartPercentile = 1 then CompletionTime else null end)   as [Top25%AverageCompletionTime]
FROM
(SELECT 
    FormName,completiontime, NTILE(4) over (partition by FormName order by completiontime) as QuartPercentile
FROM 
    FormSummaries 
WHERE CompletionTime IS NOT NULL)
    x

GROUP BY formname

請記住，如果您的completetimetime列包含整數，AVG將返回一個整數，因此您可能需要轉換以獲取所需的精度，例如

AVG( case when QuartPercentile = 1 then cast(CompletionTime AS decimal(9,2))  else null end) 

Answer 2

您可以使用CTE + PIVOT：

;WITH PercentCount AS (
SELECT  FormName,
        COUNT(*)/4 as [Bottom25Percent],
        COUNT(*) as [Top25Percent]
FROM Forms
GROUP BY FormName
), FormsWithRowNumber AS (
SELECT  f.FormName,
        f.CompletionTime,
        ROW_NUMBER() OVER (PARTITION BY f.FormName ORDER BY f.CompletionTime) as rn
FROM Forms f
), final AS (
SELECT  f.FormName, 
        f.CompletionTime,       
        CASE WHEN f.rn between 1 and [Bottom25Percent] THEN 1 
             WHEN f.rn between [Top25Percent]-[Bottom25Percent]+1 and [Top25Percent] THEN 2
             ELSE 0 END as [TopBottom]
FROM FormsWithRowNumber f
INNER JOIN PercentCount p
    ON p.FormName = f.FormName
)

SELECT *
FROM final
PIVOT (
    AVG(CompletionTime) FOR TopBottom IN ([1],[2])
) as pvt

輸出：

FormName    Top25%AverageCompletionTime Bottom25%AverageCompletionTime
Form1       15                          85
Form2       35                          560