MAX of SUMs from GROUP BY of JOIN - sql

Question

I'm stuck.
I have two tables:
First, [PurchasedItemsByCustomer] with the columns:
[CustID] INT NULL,
[ItemId] INT NULL,
[Quantity] INT NULL,
[OnDate] DATE NULL
Second, Table [Items] with the columns:
[ItemId] INT NULL,
[Price] FLOAT NULL,
[CategoryId] INT NULL
I need to output a list with 3 columns:
a month
the category which sold the most (in items quantity) in that month
how many items from that category were purchased in that month.
Thank you

Answer 1

I think you can use a query like this:
;With SoldPerMonth as (
select datepart(month, p.onDate) [Month], i.CategoryId [Category], sum(p.Quntity) [Count]
from PurchasedItemsByCustomer p
join Items i on p.ItemId = i.ItemId
group by datepart(month, p.onDate), i.CategoryId
), SoldPerMonthRanked as (
select *, rank() over (partition by [Month] order by [Count] desc) rnk
from SoldPerMonth
)
select [Month], [Category], [Count]
from SoldPerMonthRanked
where rnk = 1;
SQL Server Demo
Note: In above query by using rank() will provide all max categories if you want to return just one row use row_number() instead.

Answer 2

Divide et Impera:
with dept_sales as(
select month(ondate) as month, year(ondate) as year, category, count(*) as N -- measure sales for each month and category
from purchase join items using itemid
group by year(ondate), month(ondate), category)
select top 1 * --pick the highest
from dept_sales
where year = year(current_timestamp) -- I imagine you need data only for current year
order by N desc --order by N asc if you want the least selling category
If you don't group by year you'll get january of all the years in the same 'january' entry, so I added a filter on current year.
I used CTE for code clarity to split the phases of calculation, you can nest them if you want to.

Answer 3

Here you go,
SELECT
A.[CategoryId],
A.[Month],
A.[CategoryMonthCount]
FROM
(
SELECT
A.[CategoryId],
A.[Month],
A.[CategoryMonthCount],
RANK() OVER(
PARTITION BY A.[Month]
ORDER BY A.[CategoryMonthCount] DESC) [RN]
FROM
(
SELECT
I.[CategoryId],
MONTH(PIBC.[OnDate]) [Month],
SUM(PIBC.[Quantity]) [CategoryMonthCount]
FROM
[dbo].[PurchasedItemsByCustomer] PIBC
JOIN
[dbo].[Items] I
GROUP BY
I.[CategoryId],
MONTH(PIBC.[OnDate])
) A
) A
WHERE
A.[RN] = 1;