Filling missing months when calculating year to date - sql

Question

I have a table cumulative year todate
year month qty_ytd
2017 01 20
2017 02 30
2018 01 50
I need to fill gabs missing months in the same year till december:
Result as example:
year month qty_ytd
2017 01 20
2017 02 30
2017 03 30
.....
2017 07 30
2017 12 30
2018 01 50
2018 02 50
....
2018 12 50
How to do it? I did'nt figure out how to fill the missing months?

Answer 1

You can use cross join to generate the rows and cross apply to get the data:
select y.y, v.m, t.qty_ytd
from (select distinct year from t) y cross join
(values (1), (2), (3), (4), . . . (12)) v(m) outer apply
(select top (1) t.*
from t
where t.year = y.year and
t.month <= y.m
order by t.m desc
) t;
Assuming qty_ytd is non-decreasing, it might be more performant to use window functions:
select y.y, v.m,
max(t.qty_ytd) over (partition by y.y order by v.m) as qty_ytd
from (select distinct year from t) y cross join
(values (1), (2), (3), (4), . . . (12)) v(m) left join
t
on t.year = y.year and
t.month = v.m;

Answer 2

Another option is to compute delta, add dummy zero deltas, restore running total. I've changed source data to show more common case
create table #t
(
year int,
month int,
qty_ytd int
);
insert #t(year, month, qty_ytd )
values
(2017, 01, 20),
(2017, 02, 30),
(2018, 04, 50) -- note month
;
select distinct year, month, sum(delta) over(partition by year order by month)
from (
-- real delta
select year, month, delta = qty_ytd - isnull(lag(qty_ytd) over (partition by year order by month),0)
from #t
union all
-- tally dummy delta
select top(24) 2017 + (n-1)/12, n%12 + 1 , 0
from
( select row_number() over(order by a.n) n
from
(values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10)) a(n),
(values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10)) b(n)
) c
)d
order by year, month;