SQL Average of Sum of Calculated Attribute - mysql

Question

I am trying to take the average of the values for a category, where the rows are grouped by sub-category with a calculated sum. The Primary key of the Parent Table is the grouped attribute of the Child Table. The grouped attribute of the Parent Table is neither the primary key or in the Child Table.
Simple representation:
select Category, avg(CalculatedSum)
from ParentTable pt
inner join (
select Subcategory, sum(Quantity * Price) as 'CalculatedSum'
from ChildTable
group by Subcategory
) ct
on pt.ID = ct.Subcategory
group by Category
The actual SQL is as follows:
select c.CU_AGE_RANGE, count(*) as '# of Customers', avg(SumSales) as 'Avg of SumSales', max([Max of SumSales]) as 'Max of SumSales', min([Min of SumSales]) as 'Min of SumSales'
from Customers c
inner join (
select CUSTOMER_ID, sum(QTY_SOLD * SALES) as SumSales, max(QTY_SOLD*SALES) as 'Max of SumSales', min(QTY_SOLD*SALES) as 'Min of SumSales'
from Sales
where (SALES > 0) and (QTY_SOLD > 0) and (COST > 0)
Group by CUSTOMER_ID
) s
on c.CUSTOMER_ID = s.CUSTOMER_ID
group by c.CU_AGE_RANGE
I have tried changing the group by clause to various orders of the Category (CU_AGE_RANGE) and Subcategory (CUSTOMER_ID) but am always having the same error.
The error is that the table will always show me the SUM of the SUMS (I believe). I am assuming this is the error because the typical average in the Child Table is 250 to 1000, and the Avg(Sum()) is returning values that are roughly the number of rows per Category times the expected Sum().
I cannot post a photo due to low reputation, so please see the following Comma Delimited Results Table:
CU_AGE_RANGE,#_of_Customers,Avg_of_SumSales,Max_of_SumSales,Min_of_SumSales
NULL,125,4261665.306,433460737.7,0.0017
20-29 ,1192,1154040.907,1374037708,0.00025
30-39 ,1902,25429.52329,29426212.64,0.00015
40-49 ,2118,2418.829874,2066725,0.0001
50-59 ,2204,114625.4111,248240261.3,0.00015
60+ ,2135,160156.4341,334617675,0.0005
patrickbig,1,65.5737,12,0.06
Under 19 ,484,1431.262112,92160,0.0001
I am trying to figure out why the AVG(SUM()) is returning what seems to be the SUM(SUM()). My current hunch is that since the SUM() is of a calculated entry, the calculated value is recalculated based on the grouping in the Parent Table. So this would be:
DESIRED:
x * y for each row in Child Table
sum(x*y) for each Subcategory
Avg(sum(x/y)) for each Category of Subcategory
QTY_SOLD * SALE for each row in Sales
sum(QTY_SOLD*SALE) for each CUSTOMER_ID
avg(sum(QTY_SOLD*SALE) for each CU_AGE_RANGE group of CUSTOMER_IDs
ACTUAL:
x * y for each row in Child Table
sum(x * y) for each Subcategory
avg(sum(x * y) for each Category
avg(sum(QTY_SOLD*SALE) for each CU_AGE_RANGE
which is equal to:
sum(QTY_SOLD*SALE) for each CU_AGE_RANGE
How do I get from the current (sum of Category) to desired (avg by Category of sum of Subcategory)?

Answer 1

Your count of customers is wrong. You are counting the number of sales made, not the number of customers. Change to count( DISTINCT c.CUSTOMER_ID ) should solve it.
select c.CU_AGE_RANGE, count( DISTINCT c.CUSTOMER_ID ) as '# of Customers', avg(SumSales) as 'Avg of SumSales', max([Max of SumSales]) as 'Max of SumSales', min([Min of SumSales]) as 'Min of SumSales'
from Customers c
inner join (
select CUSTOMER_ID, sum(QTY_SOLD * SALES) as SumSales, max(QTY_SOLD*SALES) as 'Max of SumSales', min(QTY_SOLD*SALES) as 'Min of SumSales'
from Sales
where (SALES > 0) and (QTY_SOLD > 0) and (COST > 0)
Group by CUSTOMER_ID
) s
on c.CUSTOMER_ID = s.CUSTOMER_ID
group by c.CU_AGE_RANGE

Answer 2

Let's think about the sub-query first:
select Subcategory, sum(Quantity * Price) as 'CalculatedSum'
from ChildTable
group by Subcategory
Each and every record of the resulting relation is representing an aggregation of a Subcategory. Now, avg(CalculatedSum) should yield the average of the CalculatedSum values. Try to calculate sum(CalculatedSum) instead and see if there is a difference.