Filling missing records with previous existing records - sql-server

Question

I have an existing database where some logic is made by the front end application.
Now I have to make reports from that database and I'm facing to a proble of missing records which are covered on a record basis in the frontend but have issues in the report
Given the following tables:
create table #T (id int, id1 int, label varchar(50))
create table #T1 (id int, T_id1 int, A int, B int, C int)
With the values:
insert into #T values (10, 1, 'label1'), (10, 2, 'label2'), (10, 3, 'label3'), (10, 15, 'label15'), (10, 16, 'label16'), (20, 100, 'label100'), (20, 101, 'label101')
insert into #T1 values (10, 1, 100, 200, 300), (10, 15, 150, 250, 350), (20, 100, 151, 251, 351), (20, 101, 151, 251, 351)
if I make a report we can see some missing records:
select #T.id, #T.id1, #T1.A, #T1.B, #T1.C
from #T left join #T1 on #T.id1 = #T1.T_id1
result:
id id1 A B C
10 1 100 200 300
10 2 NULL NULL NULL
10 3 NULL NULL NULL
10 15 150 250 350
10 16 NULL NULL NULL
20 100 151 251 351
20 101 151 251 351
Expected result would be:
id id1 A B B
10 1 100 200 300
10 2 100 200 300
10 3 100 200 300
10 15 150 250 350
10 16 150 250 350
20 100 151 251 351
20 101 151 251 351
As you can see here the missing data is filled out of the the first (in id, id1 order) previous existing record for a given id. For a given id there can be any number of "missing" records and for the given id there can be any number of existing records after a not existing ones.
I can do this with a cursor but I'm looking for a solution without cursor

Answer 1

You can use subquery (to find groups with same values) + window function
WITH Grouped AS (
SELECT #T.id, #T.id1, #T1.A, #T1.B, #T1.C,
GroupN = SUM(CASE WHEN #T1.A IS NULL THEN 0 ELSE 1 END) OVER(/* PARTITION BY id ? */ ORDER BY id1 ROWS UNBOUNDED PRECEDING)
FROM #T
LEFT JOIN #T1 ON #T.id1 = #T1.T_id1
)
SELECT Grouped.id, Grouped.id1,
A = MAX(A) OVER(PARTITION BY GroupN),
B = MAX(B) OVER(PARTITION BY GroupN),
C = MAX(C) OVER(PARTITION BY GroupN)
FROM Grouped

Answer 2

You can use below sql for thedesired output:
with cte (id, id1, A, B, C)
as
(
select #T.id, #T.id1, #T1.A, #T1.B, #T1.C
from #T left join #T1 on #T.id1 = #T1.T_id1
)
select cte.id, cte.id1,
coalesce(cte.A,TT.A) A,
coalesce(cte.B,TT.B) B,
coalesce(cte.C,TT.C) C
from cte
left join
(
select p.id1,max(q.id1) id1_max
from cte p
inner join cte q on p.id1 > q.id1 and p.a is null and q.a is not null
group by p.id1
) T on cte.id1 = T.id1
left join cte TT on T.id1_max = TT.id1