Grouping date ranges into rolling group of 15 days - sql

Question

I have data with start and end datetime on each row, I'm trying to group all rows that fall within 15days of the end datetime, reset and begin new group using the end datetime when the start date doesn't fall in 15 days.
I tried different solutions like using logic to self join and try to pick min end date after looking for start date time between end datetime and end datetime + 30, but I'm not able to identify a way to continue using end datetime from the start/previous row when the start datetime is within 15days till it finds a row that doesn't.
with cte as
(
SELECT 1 id, '2018-12-25' col1, '2019-01-05' col2
UNION ALL
SELECT 1, '2019-03-01' col1, '2019-03-10' col2
UNION ALL
SELECT 1, '2019-03-15' col1, '2019-03-19' col2
UNION ALL
SELECT 1, '2019-03-22' col1, '2019-03-28' col2
UNION ALL
SELECT 1, '2019-03-30' col1, '2019-04-02' col2
UNION ALL
SELECT 1, '2019-04-10' col1, '2019-04-15' col2
UNION ALL
SELECT 1, '2019-04-18' col1, '2019-04-25' col2
), STG AS
(
SELECT A.*, MIN(B.COL2) AS GRP_COL2
FROm CTE A
LEFT OUTER JOIN CTE B ON A.col1 BETWEEN B.col2 AND DATEADD(day, 15, B.col2)
GROUP BY A.id, A.col1, A.col2
)
SELECT A.id, A.col1, A.col2, COALESCE(GRP_COL2, A.COL2) AS GRP_COL2
FROM STG A
ORDER By 1,2
--this returns wrong result for 04/10 and 04/18 dated rows in this case
For example I have following data:
ProductId ProductStartDt ProductEndDt
1 2018-12-25 2019-01-05
1 2019-03-01 2019-03-10
1 2019-03-15 2019-03-19
1 2019-03-22 2019-03-28
1 2019-03-30 2019-04-02
1 2019-04-10 2019-04-15
1 2019-04-18 2019-04-25
So what I'm trying to do is start from min end date and group all rows with start date that fall in end date + 15 days range as one group and start next row as new group and continue using the end date of that row as new start for group.
Expected Output
ProductId ProductStartDt ProductEndDt GroupNo
1 2018-12-25 2019-01-05 1
1 2019-03-01 2019-03-10 2
1 2019-03-15 2019-03-19 2
1 2019-03-22 2019-03-28 2
1 2019-03-30 2019-04-02 3
1 2019-04-10 2019-04-15 3
1 2019-04-18 2019-04-25 4
For group 2, the start date doesn't fall within 15days of the end date from Group 1 or start row and this will trigger start of new rows. We see that 03/15 and 03/22 are within 15 days of 03/10 and we group them into single group. 03/30 doesn't fall in the previous group and this will cause start of new group 3 till we find all start dates within the end datetime + 15days.

Answer 1

With recursive CTE we can iterate over initial table, and decide whether current row belongs an existing group or it's a new group:
;with Product as (
select * from (
VALUES
(1, '2018-12-25','2019-01-05' ),
(1, '2019-03-01','2019-03-10' ),
(1, '2019-03-15','2019-03-19' ),
(1, '2019-03-22','2019-03-28' ),
(1, '2019-03-30','2019-04-02' ),
(1, '2019-04-10','2019-04-15' ),
(1, '2019-04-18','2019-04-25' )
) as a1 (ProductId ,ProductStartDt ,ProductEndDt)
), OrderedProduct as (
select *, ROW_NUMBER() over (order by ProductStartDt) as RowNum
from Product
), DateGroupsInterim (RowNum, GroupNum, GrpStartDt) as (
select RowNum, 1, ProductEndDt
from OrderedProduct
where RowNum=1
union all
select OrderedProduct.RowNum,
CASE WHEN OrderedProduct.ProductStartDt <= dateadd(day, 15, dgi.GrpStartDt)
THEN dgi.GroupNum
ELSE dgi.GroupNum + 1
END,
CASE WHEN OrderedProduct.ProductStartDt <= dateadd(day, 15, dgi.GrpStartDt)
THEN dgi.GrpStartDt
ELSE OrderedProduct.ProductEndDt
END
from DateGroupsInterim dgi
join OrderedProduct on OrderedProduct.RowNum=dgi.RowNum+1
) select OrderedProduct.ProductId, OrderedProduct.ProductStartDt, OrderedProduct.ProductEndDt, DateGroupsInterim.GroupNum as GroupNo
from DateGroupsInterim
JOIN OrderedProduct on OrderedProduct.RowNum = DateGroupsInterim.RowNum;