Calculating missing months - sql

Question

I use the following query in ordre to filling missing months
Declare #Sample Table(year int, month int,product as nvarchar(50), qty_ytd int);
Insert #Sample(year, month, qty_ytd) Values
(2017, 01,'book', 20),
(2017, 02, 'pc', 30),
(2018, 01, 'book', 50);
;With Months As
(Select 1 As month
Union All
Select month + 1 From Months Where month < 12)
, YearsAndMonths As
(Select distinct year,m.month from #Sample cross join Months m)
select ym.*, coalesce(s.qty_ytd, s2.qty_ytd) qty_ytd, coalesce(s.qty_ytd, 0) QTY from YearsAndMonths ym
left join #sample s on ym.year = s.year and ym.month = s.month
left join (select qty_ytd, year,
row_number() over (partition by year order by month desc) rn
from #Sample) s2 on ym.year = s2.year and rn = 1
How could I add 'product ' as well ?

Answer 1

Firstly, I would recommend creating a calendar table since this pops up as a use case every once in a while. A quick example can be found here
Now, once you have the calendar table (let's call it static.calendar) ready, the code is fairly simple as follows:
with Products
as
(
SELECT distinct product
FROM #Sample
),
TimeRange
as
(
SELECT DISTINCT year,
month
FROM static.calendar
)
ProductTimeRange
as
(
SELECT p.products,
tr.year,
tr.month
FROM Products as p
CROSS JOIN TimeRange as tr
)
SELECT ptr.products,
ptr.year,
ptr.month,
s.qty_ytd
FROM ProductTimeRange as ptr
LEFT JOIN #sample as s
ON ptr.products = s.products
AND ptr.year = s.year
AND ptr.month = s.month
ORDER BY ptr.products,
ptr.year,
ptr.month

Answer 2

Use a cross join to generate the rows that you want -- all the years, months, and products.
Then use left join to bring in the data you want:
With Months As (
Select 1 As month
Union All
Select month + 1
From Months
Where month < 12
)
select y.year, m.month, s.product, coalesce(qty_ytd, 0) as qty_ytd
from (select distinct year from #sample) y cross join
months m cross join
(select distinct product from #sample) p left join
#sample s
on s.year = y.year and s.month = m.month and s.product = p.product;
Here is a db<>fiddle.