Sql计算平均值

Question

I am trying to calculate the average number of pests in a Town who are using the same Variety of crops..

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

But this query keeps on giving me the syntax error ie Invalid Column name PestPopulation1 where as i thoroughly checked the spellings of my column name... PestPopuplation is of varchar(max) type

Following is my query

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

The reason this gives you an error is because you gave the expression cast(PestPopulation1 as float) an alias Pests .

You should use that alias in your outer query, like this:

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

You should be able to simplify this query by pushing the cast into avg , like this:

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

Error converting data type nvarchar to float . My PestPopulation1 column in of varchar(max) .

You need to use CONVERT instead of CAST (see above).

Answer 2

As mentioned, the problem is that you refer to a column that's not available in that scope.

Altogether there's not even a good reason to use such a contrived query. This should work just as well, and is more readable:

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 3

try this

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 4

Why not just (with CONVERT instead of CAST):

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

The error in your query is that you are referencing a column that the table T has not. The derived table T has columns DistrictName, TownName,VarietyOfCrop and Pests, and you are referencing the column PestPopulation1 that doesn't exists there.

Answer 5

You are using the wrong column name, the alias for PestPopulation1 is Pests in your subquery, please find the corrected query:

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