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calculating a weighted average in SQL, SSRS

Posted on 2010-11-14
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Last Modified: 2012-05-10
Hello Experts,

Does anyone know how to calculate a weighted average in SQL?

First I get the average of something: COUNT(transactions)/Fee = Average
Then to get the weighted average I would have to total the above values of 'Average' for the three different transactions types we use, and then divide that by the total count.

I cannot create a calculated field because in SSRS you can't use aggregate functions in a calc field. Also, in SSRS, I have tried creating an expression:

=(SUM((Fields!CRDTAMNT.Value/COUNT(Fields!Trans.Value))* COUNT(Fields!Trans.Value)))/COUNT(Fields!Trans.Value)

and I keep getting errors about nested aggregate functions.

So first of all, should I do this in my query or somewhere in SSRS?
Second of all, what is the code/syntax to achieve this?

Thanks.

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Question by:erp1022
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7 Comments
 
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Expert Comment

by:arnold
ID: 34131907
You have the value and the weight in each row or you know the weights.
in either case you would first sum the weights into a variable and then use the formula of wight*value/sum(weights)

select @total_weight=sum(weights) from table;
select @weighted_average+=sum(weights*values/@total_weight) from table;

have not tried it such that not sure whether it is even valid.
 
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Assisted Solution

by:Patrick Matthews
Patrick Matthews earned 200 total points
ID: 34131921
I would create the weighted average in your underlying query.

This article shows how to calculate a weighted average; while it is done for Access, the logic for SQL Server is much the same, except you would use CASE instead of IIf:

SELECT {group by columns if desired,} (CASE WHEN MIN([WeightColumn]) >=0 AND
    MAX([WeightColumn]) > 0 THEN SUM([WeightColumn] * [ValueColumn]) ELSE NULL END) /
    SUM([WeightColumn]) AS WtdAvg
FROM {tables}
{GROUP BY {group by columns}}

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Patrick
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Author Comment

by:erp1022
ID: 34132408
Okay now I'm really confused.

So if my weighted average is based on a count of transactions [COUNT(Trans)] * an amount, how would I write my code in SQL?
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LVL 92

Expert Comment

by:Patrick Matthews
ID: 34132429
Perhaps it would be best if you could work up some sample source data, and then the output you would expect given that input.
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Author Comment

by:erp1022
ID: 34132779
Please see attachment...
Weighted-Average.xls
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LVL 41

Expert Comment

by:ralmada
ID: 34136671
try this query
select 	transtype,
	count(items),
	sum(amount),
	sum(amount * items) * 1.0 / sum(amount) as weight_avg
from yourtable
group by transtype
with rollup

Open in new window

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LVL 41

Accepted Solution

by:
ralmada earned 300 total points
ID: 34136969
sorry actually it should be
select 	transtype,
	count(items),
	sum(amount),
	sum(amount) * count(items) * 1.0 / sum(amount) as weight_avg
from yourtable
group by transtype
with rollup

Open in new window

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