Use Calendar table to generate historical view of the data - sql

Question

I have a created_date (timestamp) on 1 of my tables, that also has the duration column of a project, and I need to join with another table that only has first_day_of_month column that has the first day of each month, and other relevant information.
Table 1
id project_id created_date duration
1 12345 01/01/2015 10
2 12345 20/10/2015 11
3 12345 10/04/2016 13
4 12345 10/08/2016 15
Table 2
project_id month_start_date
12345 01/01/2015
12345 01/02/2015
12345 01/03/2015
12345 01/04/2015
...
12345 01/08/2016
Expected result
project_id month_start_date duration
12345 01/01/2015 10
12345 01/02/2015 10
...
12345 01/10/2015 11
12345 01/11/2015 11
...
12345 01/04/2016 13
12345 01/05/2016 13
12345 01/06/2016 13
...
12345 01/08/2016 15
I want to be able to present the data listed in my second table historically. So, basically I want the query to return the same duration related to the month_start_date, so that values will repeat until another dateadd(month,datediff(month,0,created_date),0) = first_day_of_month is met... and so forth.
This is my query:
select table2.project_name,
table2.month_start_date,
table1.duration,
table1.created_date
from table1 left outer join table2
on table1.project_id=table2.project_id
where dateadd(month,datediff(month,0,table1.created_date),0)<=table2.month_start_date
group by table2.project_name,table2.month_start_date,table1.duration,table1.created_date
order by table2.month_start_date asc
but I get repeated records on this:
Result I'm getting
project_id month_start_date duration
12345 01/01/2015 10
12345 01/02/2015 10
...
12345 01/10/2015 10
12345 01/10/2015 11
...
12345 01/04/2016 10
12345 01/04/2016 11
12345 01/04/2016 13
...
12345 01/08/2016 10
12345 01/08/2016 11
12345 01/08/2016 13
12345 01/08/2016 15
Can anyone help?
Thank you!

Answer 1

I'd use CROSS/OUTER APPLY operator.
Here is one possible variant. For each row in your calendar table Table2 (for each month) the inner correlated subquery inside the CROSS APPLY finds one row from Table1. It will be the row with the same project_id and the first row with created_date before the month_start_date plus 1 month.
SELECT
Table2.project_id
,Table2.month_start_date
,Durations.duration
FROM
Table2
CROSS APPLY
(
SELECT TOP(1) Table1.duration
FROM Table1
WHERE
Table1.project_id = Table2.project_id
AND Table1.created_date < DATEADD(month, 1, Table2.month_start_date)
ORDER BY Table1.created_date DESC
) AS Durations
;
Make sure that Table1 has index on (project_id, created_date) include (duration). Otherwise, performance would be poor.