ISNULL() for calculated column in Full Join. SQL Server - sql-server

Question

I'm currently facing a problem when calculating the standard deviation in an SQL Server statement.
My problem: I have two tables.
T1:
Reg_Month
---------
1
2
3
...
T2:
Product Month Consumption
-------------------------------
ProdA 1 200
ProdB 1 10
ProdA 1 300
ProdC 2 100
ProdA 2 200
...
Now what I want is something like this, for calculating my STDEV over a year:
Reg_Month Product Month Sum
---------------------------------
1 ProdA 1 500
1 ProdB 1 10
1 ProdC 1 0
2 ProdA 2 200
2 ProdB 2 0
2 ProdC 2 0
So now I don't need this table to be set up, but I need to calculate the STDEV and AVG of the column "Sum" for each Product. The Problem is to include the NULLS.
This gives me the table I want for ProdA, but with NULLS whenever there was no consumption:
SELECT *
FROM T1
FULL JOIN (SELECT Product, Month, SUM(Consumption) AS Sum,
FROM T2
WHERE (Product = 'ProdA')
GROUP BY Month, Product) sub ON (T1.Reg_Month = T2.Month)`
This gives me the STDEV:
SELECT Stdev(Sum)
FROM
(SELECT *
FROM T1
FULL JOIN (SELECT Product, Month, SUM(Consumption) AS Sum,
FROM T2
WHERE (Product = 'ProdA')
GROUP BY Month, Product) sub ON (T1.Reg_Month = T2.Month)) sub
WHERE Product = 'ProdA'`
But the problem is, that it doesn't give me the correct STDEV for the entire year, if there is a month in which there was no consumption, because the NULLs (that appear due to the join) are ignored.
My approaches:
ISNULL():
SELECT Stdev(Sum)
FROM
(SELECT *
FROM T1
FULL JOIN (SELECT Product, Month, ISNULL(SUM(Consumption), 0) AS Sum,
FROM T2
WHERE (Product = 'ProdA')
GROUP BY Month, Product) sub ON (T1.Reg_Month = T2.Month)) sub
WHERE Product = 'ProdA'`
doesn't work (maybe because the null is generated after the join?)
CASE:
SELECT Stdev(Sum)
FROM
(SELECT *
FROM T1
FULL JOIN (SELECT Product, Month, CASE WHEN SUM(Consumption) IS NULL THEN 0 ELSE Sum(Consumption) END AS Sum,
FROM T2
WHERE (Product = 'ProdA')
GROUP BY Month, Product) sub ON (T1.Reg_Month = T2.Month)) sub
WHERE Product = 'ProdA'
doesn't work (probably the same reason)
I hope I was able to illustrate my example properly. Now, do you have any idea how get the right results for the STDEV?
Your input would be greatly appreciated!
Thanks a lot,
Clemens

Answer 1

if you are only doing one product at a time
SELECT sum(isnull(T2month.MonthSum ,0))
, Stdev(isnull(T2month.MonthSum ,0))
FROM T1
LEFT JOIN
(select Month, sum(Consumption) as MonthSum
from T2
where Product = 'ProdA'
group by Month) T2month
ON T1.Reg_Month = T2month.Month
for all products
SELECT Product.Name
, sum(isnull(T2month.MonthSum ,0))
, Stdev(isnull(T2month.MonthSum ,0))
FROM T1
cross apply
(values ('ProdA'), ('ProdB'), ('ProdC')) Product(Name)
LEFT JOIN
(select Product, Month, sum(Consumption) as MonthSum
from T2
group by Product, Month) T2month
ON T1.Reg_Month = T2month.Month
AND T2month.Product = Product.Name
Group By Product.Name
Hey OP left join does NOT leave out periods
That is what a left join does
select lj.val, isnull(jj.val,0)
from ( values (1), (2), (3), (4) ) lj(val)
left join ( values (1), (2), (4) ) jj(val)
on lj.val = jj.val
order by lj.val