Get dates with in consecutive 50 days of given date - tsql

Question

Activity table:
create table #activity(id int, begin_date datetime, end_date datetime)
insert into #activity values(1, '1/1/2017', '1/31/2017')
insert into #activity values(1, '9/1/2017', '9/15/2017')
insert into #activity values(1, '4/1/2017', '4/15/2017')
insert into #activity values(1, '2/5/2017', '2/15/2017')
insert into #activity values(1, '8/1/2017', '8/31/2017')
Insert into #activity values(2, '11/1/2016', '11/15/2016')
Now input date is 12/1/2016 and id, would like to get all activities within 50 days after 12/1/2016. Query should return activities with begin dates 1/1/2017, 2/5/2017 (because this is within 50 days of 1/31/2017), and 4/1/17.
8/1/2017 and 9/1/2017 of id 1 shouldn't be selected 8/1 is not with in 50 days of 4/15 and 50 day cycle was broken.
TIA

Answer 1

The OP says:
would like to get all activities within 50 days after 12/1/2016
one possible query to achieve that result would be
-- get all activities with a begin_date within 50 days of input_date
select *
from #activity as a
where #input_date <= a.begin_date and a.begin_date < dateadd(day, 50, #input_date)
However, the OP then says:
Query should return activities with begin dates 1/1/2017, 2/5/2017 (because this is within 50 days of 1/31/2017), and 4/1/17. 8/1/2017 and 9/1/2017 of id 1 shouldn't be selected 8/1 is not with in 50 days of 4/15 and 50 day cycle was broken.
This would indicate that you want to find all sequential activities starting after 12/1/2016 where the gap between sequential activities is less than 50 days.
One possible approach for this is to use the lag function. An example of how to use the lag function on this problem is:
select
a.*
, lag(a.end_date, 1, #input_date) over (order by a.end_date) as previous_end
, datediff(day, lag(a.end_date, 1, #input_date) over (order by a.end_date), a.begin_date) as previous_end_to_this_begin
from #activity as a
where #input_date <= a.begin_date
order by a.begin_date
Simplifying that slightly would produce this:
-- get all activities in a row where the gap between activities is less than 50
select * from #activity as a where #input_date <= a.begin_date and a.begin_date < (
select
min(a.begin_date) as first_begin_to_not_include
from
(
select
a.begin_date
, datediff(day, lag(a.end_date, 1, #input_date) over (order by a.end_date), a.begin_date) as previous_end_to_this_begin
from #activity as a
where #input_date <= a.begin_date
) as a
where a.previous_end_to_this_begin > 50
)
order by a.begin_date