Group records when date is within N minutes - sql

Question

It's not as simple as creating intervals of time that are N minutes long. One record might be 10:04, and the other 10:17 where N is 15.
Perhaps a user-function will work, maybe a CTE. It could require multiple joins on the same source table.
I'm looking for the most "elegant" solution. Maybe there's a feature in SQL I didn't know about which makes this easy.
Here is a reference scenario to make answers more consistent with each other:
create table Comparisons (
DateField DateTime NOT NULL,
Amount int not null, -- default to 5
)
insert into Comparisons (DateField) values ('2000-01-01 10:04'),('2000-01-01 10:17'),
('2000-01-01 12:01'),('2000-01-01 11:54'),('2000-01-01 03:02'),('2000-01-01 03:05'),
('2000-01-01 05:02'),('2000-01-01 05:05'),('2000-01-01 05:19')
output expected:
min: .. 10:04, max: .. 10:17, sum: 10
min: .. 11:54, max: .. 12:01, sum: 10
min: .. 03:02, max: .. 03:05, sum: 10
min: .. 05:02, max: .. 05:19, sum: 15 [optional]
The last output is optional, but if an elegant solution has that as a side-effect, it's acceptable. If an elegant solution can't achieve that optional last output, it won't be a deal breaker.

Answer 1

I believe this produces the results you want:
DECLARE #Comparisons TABLE (i DATETIME, amt INT NOT NULL DEFAULT(5));
INSERT #Comparisons (i) VALUES ('2016-01-01 10:04:00.000')
, ('2016-01-01 10:17:00.000')
, ('2016-01-01 10:25:00.000')
, ('2016-01-01 10:37:00.000')
, ('2016-01-01 10:44:00.000')
, ('2016-01-01 11:52:00.000')
, ('2016-01-01 11:59:00.000')
, ('2016-01-01 12:10:00.000')
, ('2016-01-01 12:22:00.000')
, ('2016-01-01 13:00:00.000')
, ('2016-01-01 09:00:00.000');
DECLARE #N INT = 15;
WITH T AS (
SELECT i
, amt
, CASE WHEN DATEDIFF(MINUTE, previ, i) <= #N THEN 0 ELSE 1 END RN1
, CASE WHEN DATEDIFF(MINUTE, i, nexti) > #N THEN 1 ELSE 0 END RN2
FROM #Comparisons t
OUTER APPLY (SELECT MAX(i) FROM #Comparisons WHERE i < t.i)x(previ)
OUTER APPLY (SELECT MIN(i) FROM #Comparisons WHERE i > t.i)y(nexti)
)
, T2 AS (
SELECT CASE RN1 WHEN 1 THEN i ELSE (SELECT MAX(i) FROM T WHERE RN1 = 1 AND i < T1.i) END mintime
, CASE WHEN RN2 = 1 THEN i ELSE ISNULL((SELECT MIN(i) FROM T WHERE RN2 = 1 AND i > T1.i), i) END maxtime
, amt
FROM T T1
)
SELECT mintime, maxtime, sum(amt) total
FROM T2
GROUP BY mintime, maxtime
ORDER BY mintime;
It's probably a little clunkier than it could be, but it's basically just grouping anything within an #N-minute chain.

Answer 2

It looks like you want to group records based on gaps between them of at least <N> minutes.
In SQL Server 2012+, you would use lag() to identify when groups start and cumulative sum to identify the groups:
select min(datefield), max(datefield), count(*) as num, sum(amount)
from (select c.*,
sum(case when prev_datefield < dateadd(minute, -N, datefield)
then 1 else 0
end) over (order by datefield) as grp
from (select c.*,
lag(datefield) over (order by datefield) as prev_datefield
from Comparisons c
) c
) c
group by grp;
In earlier versions you can use correlated subqueries or apply for the same functionality (albeit at much worse performance).

Answer 3

Intervals could be used, if adjacent intervals are checked. This would require multiplying the source table records by 3
Pseudo-code:
select *
from Comparisons C, {-1, 0, 1} M
group by (datediff(mi, C.DateField, 0) / N) + M
The problem with this approach is how to eliminate the extra results. I suspect this is a deadend approach but someone else might see value in it.
Update: This approach would not work with the 4th expected output [min: .. 05:02, max: .. 05:19, sum: 15]