Group dates by considering look ahead days - sql-server

Question

I am using SQL Server 2017. I have a table Requests, to make things simple, there's only one column RequestDate. For example,
RquestDate
4/11
4/12
4/13
4/16
4/18
I need to group by RequestDate by considering look ahead days. If look ahead day is 0, the result should be the same as raw table.
If look ahead day is 1, it means when I look at 4/11, I need to check if 4/12 exists, if so, group 4/12 into 4/11.
The result is:
4/11 --it groups 4/12
4/13
4/16
4/18
If look ahead day is 2, when looking at 4/11, it groups 4/12, 4/13 into it.
The result is:
4/11 -- group 4/12 and 4/13.
4/16 -- group 4/18
So this problem is different from the typical gap and island problem. Because when group dates, there could be gap there, e.g, when look ahead day is 2, 4/16 groups 4/17 and 4/18.
I tried some ways but can't find a decent solution.

Answer 1

A recursive common table expression could work.
Select start request date using a min() function.
Use that same date as the grouping start date.
Step 1 and 2 make up the recursion anchor / start row.
Recursively go looking for the next request date. This date is higher than the previous date (r.RequestDate > c.RequestDate) and does not have another row
that follow the same criteria before it (not exists ... r2.RequestDate < r.RequestDate).
If the current request date (from step 3) falls within the look ahead interval length, then maintain the grouping start date (then c.RequestGroupDate), otherwise start a new group on the current request date (else r.RequestDate).
Step 3 and 4 make up the recursive part of the CTE.
After the recursion every request date as a corresponding request grouping date. The group by r.RequestGroupDate clause reduces the result output to the distinct values.
Sample data
create table Requests
(
RequestDate date
);
insert into Requests (RequestDate) values
('2021-04-11'),
('2021-04-12'),
('2021-04-13'),
('2021-04-16'),
('2021-04-18');
Solution
declare #lookAhead int = 1; -- look ahead days parameter
with rcte as
(
select min(r.RequestDate) as RequestDate,
min(r.RequestDate) as RequestGroupDate
from Requests r
union all
select r.RequestDate,
case
when datediff(day, c.RequestGroupDate, r.RequestDate) <= #lookAhead
then c.RequestGroupDate
else r.RequestDate
end
from rcte c
join Requests r
on r.RequestDate > c.RequestDate
where not exists ( select 'x'
from Requests r2
where r2.RequestDate > c.RequestDate
and r2.RequestDate < r.RequestDate )
)
select r.RequestGroupDate
from rcte r
group by r.RequestGroupDate;
Result
For #lookAhead = 1:
RequestGroupDate
----------------
2021-04-11
2021-04-13
2021-04-16
2021-04-18
For #lookahead = 2:
RequestGroupDate
----------------
2021-04-11
2021-04-16
Fiddle to see things in action.