的Sql計算平均

Question

我試圖計算害蟲的誰使用作物的同一品種一個城市的平均數量..

DistrictName | TownName   | CropName |Pests 
Rawalpindi  | pindi    | CIM 506  | 3.3  
Rawalpindi  | pindi    | CIM 506  | 3.8  
Rawalpindi  | Muree    | CIM 506  | 3.5 

但此查詢不斷給我的語法錯誤，即無效列名PestPopulation1凡爲我徹底檢查我的列名的拼寫... PestPopuplation是varchar(max)型

下面的是我的查詢

SELECT T.DistrictName, T.TownName, T.VarietyOfCrop, avg(T.PestPopulation1) 
FROM (
    SELECT DistrictName, TownName,VarietyOfCrop, cast(PestPopulation1 as float) as Pests 
    FROM FinalTable2 
) as T 
group by T.DistrictName, T.TownName, T.VarietyOfCrop 

Answer 1

如前所述，問題在於您引用了該範圍內不可用的列。

總的來說，甚至沒有一個很好的理由來使用這樣一個人爲的查詢。這應該工作一樣好，而且是更具可讀性：

SELECT T.DistrictName, T.TownName, T.VarietyOfCrop, avg(cast(T.Pests as float)) 
FROM FinalTable2 T 
group by T.DistrictName, T.TownName, T.VarietyOfCrop 

Answer 2

試試這個

SELECT T.DistrictName, T.TownName, T.VarietyOfCrop, avg(T.PestPopulation1) 
FROM (
    SELECT DistrictName, TownName,VarietyOfCrop, cast(Pests as float) as PestPopulation1 
    FROM FinalTable2 
) as T 
group by T.DistrictName, T.TownName, T.VarietyOfCrop 

Answer 3

這給你一個錯誤的原因是因爲你給了表達cast(PestPopulation1 as float)別名Pests。

您應該使用別名在外部查詢，像這樣：

SELECT T.DistrictName, T.TownName, T.VarietyOfCrop, avg(T.Pests) 
FROM (
    SELECT DistrictName, TownName,VarietyOfCrop, cast(PestPopulation1 as float) as Pests 
    FROM FinalTable2 
) as T 
group by T.DistrictName, T.TownName, T.VarietyOfCrop 

您應該能夠通過推動cast簡化此查詢到avg，像這樣：

SELECT 
    DistrictName 
, TownName 
, VarietyOfCrop 
, avg(convert(float, PestPopulation1)) as avg_pests 
FROM FinalTable2 
group by T.DistrictName, T.TownName, T.VarietyOfCrop 

將數據類型nvarchar轉換爲float時出錯。我的PestPopulation1列中的varchar(max)。

您需要使用CONVERT而不是CAST（參見上文）。

Answer 4

爲什麼不（與CONVERT而不是CAST）：

SELECT DistrictName, TownName,VarietyOfCrop, AVG(CONVERT(float, PestPopulation1)) as Pests 
FROM FinalTable2 
GROUP BY DistrictName, TownName,VarietyOfCrop 

查詢中的錯誤是，你引用的列的表T還沒有。派生表T具有ColumnName，TownName，VarietyOfCrop和Pests列，並且引用的列PestPopulation1不存在。

Answer 5

您使用了錯誤的列名，別名，PestPopulation1是在你的子查詢害蟲，請找修正查詢：

SELECT T.DistrictName, T.TownName, T.VarietyOfCrop, avg(T.Pests) 
FROM ( 
SELECT DistrictName, TownName,VarietyOfCrop, cast(PestPopulation1 as float) as Pests 
FROM FinalTable2 
) as T 
group by T.DistrictName, T.TownName, T.VarietyOfCrop