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:

1. 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';
   

2. 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
   

3. 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.