How to find overlapping date ranges from same tables (multiple overlapping) - sql

Question

there are user data with data range
I'm trying to update flag section by using query. If same user id group overlap any single day range, it should say overlapping in flag section.
can you please give me some idea how to flag this overlapping data
Thanks
Text file format:
ID UserID registereddate termdate flag
1 abcd 1/1/2018 2/28/2018 overlapping with 2
2 abcd 1/1/2018 6/30/2018 overlapping with 1
3 abcd 8/1/2018 12/31/2018
4 bbbb 5/1/2018 6/30/2018 overlapping with 5
5 bbbb 6/1/2018 7/30/2018 overlapping with 4
6 bbbb 9/1/2018 9/30/2018
7 bbbb 10/1/2018 10/30/2018
8 bbbb 11/1/2018 11/30/2018
9 ccccc 7/1/2018 9/30/2018 overlapping with 10
10 ccccc 9/1/2018 12/31/2018 overlapping with 9
11 dddd 8/1/2018 8/31/2018
12 dddd 12/1/2018 12/31/2018
13 eeee 9/1/2018 12/31/2018 overlapping with 17
14 eeee 8/1/2018 8/31/2018
15 eeee 9/1/2018 9/30/2018 overlapping with 15

Answer 1

To get 'overlapping', use exists:
select t.*,
(case when exists (select 1
from t t2
where t2.registereddate < t.termdate and
t2.termdate > t.registereddate
)
then 'overlaps'
end)
from t;
In an update, this looks like:
update t
set flag = 'overlaps'
where exists (select 1
from t t2
where t2.registereddate < t.termdate and
t2.termdate > t.registereddate
);
Getting the list of overlapping records in a string field is much, much more complicated in SQL Server. Getting a pairwise list of overlaps is pretty easy.

Answer 2

You could also inner join the table onto itself and compare the dates that way
SELECT *
FROM [Table] t1
INNER JOIN [Table] t2
ON t1.ID <> t2.ID
AND t1.UserId = t2.UserId
AND ((t1.RegisterDate BETWEEN t2.RegisterDate AND t2.TermDate) OR (t1.TermDate BETWEEN t2.RegisterDate AND t2.TermDate))
Although the more field you end up having the more complicated this becomes
See this DBFiddle