的Sql计算平均

Question

我试图计算害虫的谁使用作物的同一品种一个城市的平均数量..

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

但此查询不断给我的语法错误，即无效列名PestPopulation1凡为我彻底检查我的列名的拼写... PestPopuplation是varchar(max)型

下面的是我的查询

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，问题在于您引用了该范围内不可用的列。

总的来说，甚至没有一个很好的理由来使用这样一个人为的查询。这应该工作一样好，而且是更具可读性：

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

试试这个

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

这给你一个错误的原因是因为你给了表达cast(PestPopulation1 as float)别名Pests。

您应该使用别名在外部查询，像这样：

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 

您应该能够通过推动cast简化此查询到avg，像这样：

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

将数据类型nvarchar转换为float时出错。我的PestPopulation1列中的varchar(max)。

您需要使用CONVERT而不是CAST（参见上文）。

Answer 4

为什么不（与CONVERT而不是CAST）：

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

查询中的错误是，你引用的列的表T还没有。派生表T具有ColumnName，TownName，VarietyOfCrop和Pests列，并且引用的列PestPopulation1不存在。

Answer 5

您使用了错误的列名，别名，PestPopulation1是在你的子查询害虫，请找修正查询：

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