How to group by Date Range starting from initial date - sql

Question

I have the following table structure
Key int
MemberID int
VisitDate DateTime
How can group all the dates falling with a given date range say 15 days..The first visit for the sameMember should be considered as the starting date.
eg
Key ID VisitDate(MM/dd/YY)
1 1 02/01/11
2 1 02/09/11
3 1 02/12/11
4 1 02/17/11
5 2 02/03/11
6 2 02/19/11
In this case the result should be
ID StartDate EndDate
1 02/01/11 02/12/11
1 02/17/11 02/17/11
2 02/03/11 02/03/11
2 02/19/11 02/19/11

Answer 1

One way to do this would be to use window aggregating. Here's how:
Setup:
DECLARE #data TABLE (
[Key] int, ID int, VisitDate date
);
INSERT INTO #data ([Key], ID, VisitDate)
SELECT 1, 1, '02/01/2011' UNION ALL
SELECT 2, 1, '02/09/2011' UNION ALL
SELECT 3, 1, '02/12/2011' UNION ALL
SELECT 4, 1, '02/17/2011' UNION ALL
SELECT 5, 2, '02/03/2011' UNION ALL
SELECT 6, 2, '02/19/2011';
Query:
WITH marked AS (
SELECT
*,
Grp = DATEDIFF(DAY, MIN(VisitDate) OVER (PARTITION BY ID), VisitDate) / 15
FROM #data
)
SELECT
ID,
StartDate = MIN(VisitDate),
EndDate = MAX(VisitDate)
FROM marked
GROUP BY ID, Grp
ORDER BY ID, StartDate
Output:
ID StartDate EndDate
----------- ---------- ----------
1 2011-02-01 2011-02-12
1 2011-02-17 2011-02-17
2 2011-02-03 2011-02-03
2 2011-02-19 2011-02-19
Basically, for each row, the query is calculating the difference of days between VisitDate and the first VisitDate for the same ID and divides it by 15. The result is then used as a grouping criterion. Note that SQL Server uses integer division when both operands of the / operator are integers.