SQL - Comparing and Grouping Data on multiple rows - sql

Question

I'm trying to query my database to find which products sold less in October than in either November or December.
I thought something like below would do it but I have a feeling the sub query will be returning the mininimum quantity for the whole database rather than for the specific product.
There must be some way of doing this using GROUP BY but I cant figure it out.
SELECT Category, Product
FROM Sales
WHERE SaleQuantity < (SELECT MIN(SaleQuantity)
FROM Sales
WHERE MonthNumber > 10)
AND MonthNumber = 10
Data looks like:
Category Product MonthNumber SaleQuantity
---------- ----------- ------------- -----------
11 14 10 210
11 14 11 200
11 14 12 390
15 12 10 55
15 12 11 24
17 12 12 129
19 10 10 12
Thanks.

Answer 1

try something like this
SELECT Category,
Product,
SUM( s.SaleQuantity ) AS saleOcotber,
SUM( ISNULL( son.SaleQuantity, 0 ) ) AS saleNovember,
SUM( ISNULL( sod.SaleQuantity, 0 ) ) AS saleDecember
FROM Sales s
LEFT OUTER JOIN Sales son ON son.Category = s.Category
AND son.Product = s.Product
AND son.MonthNumber = 11
LEFT OUTER JOIN Sales sod ON sod.Category = s.Category
AND sod.Product = s.Product
AND sod.MonthNumber = 11
WHERE s.MonthNumber = 10
GROUP BY Category,Product
WHERE SUM( s.SaleQuantity ) < SUM( ISNULL( son.SaleQuantity, 0 ) )
OR SUM( s.SaleQuantity ) < SUM( ISNULL( sod.SaleQuantity, 0 ) )
I have no tested this select but i think it will do the job if there is something not clear
please ask
Best Regards,
Iordan
PS. I presume you are using some version of MSSQL if not try to rewrite it by yourself int SQL you are using

Answer 2

Your table already appears to be summarised by Category, Product and MonthNumber, for SalesQuantity. If so, try this:
select distinct Category, Product
from Sales s11_12
where MonthNumber in (11,12) and
not exists (select null
from Sales s10
where s10.Category = s11_12.Category and
s10.Product = s11_12.Product and
s10.SalesQuantity >= s11_12.SalesQuantity)