TSQL - Recursively select date between 2 given dates

Question

I have table in my db called Tasks. Every record in that table has 2 fields: StartDate, EndDate
I need to create recursive stored procedure that will send mails in middle of those dates.
For example:
Start is 2013-10-22 12:00:00:000
End is 2013-10-24 12:00:00:000

I can do:

SELECT DATEADD(ms, 
       DATEDIFF(ms,'2013-10-22 12:00:00:000', '2013-10-24 12:00:00:000')/2,
       '2013-10-22 12:00:00:000')

and then check if now is greater than that date, if Yes then I can send mail.

But I need to do that recursively: first main must be send in middle, second in 1/4, third in 1/8 etc and at the last when there are 2 hours left.

My first idea was to add column to my table and store date of last main in it, but I would like to avoid modifying tables.

I think it recursive select would be better, but any ideas on how to solve that are welcome :)

EDIT: My sample fiddle: http://sqlfiddle.com/#!3/25d0d/1

My example:
task starts at 2013-10-22 8:00 and ends at 2013-10-22 21:00

procedure starts as 2013-10-22 10:00
first record has send time 14:30 so nothing to send

procedure starts as 2013-10-22 12:00
first record has send time 14:30 so nothing to send

procedure starts as 2013-10-22 14:00
first record has send time 14:30 so nothing to send

procedure starts as 2013-10-22 16:00
first record has send time 14:30 so send mail about that task

next message should be send about 17:45

procedure starts as 2013-10-22 18:00

first record has send time 17:45 so send mail about that task

next message should be send about 19:22

procedure starts as 2013-10-22 20:00
first record has send time 19:22 so mail should be sended,
but because from 19:22 till 21:00 is less that 2 hours no mail is needed

Answer 1

Can't test on SQL Server 2005, but on SQL Server 2008 you can use a recursive common table expression (replacing the fixed dates below with your procedure parameters). The first part gets the first time, the second part keeps calculating the time between the last time and the end time until the difference between the times is less than 4 hours;

WITH cte AS (
  SELECT DATEADD(ms, 
                 DATEDIFF(ms, '2013-10-22 12:00:00:000', 
                              '2013-10-24 12:00:00:000')/2,
                              '2013-10-22 12:00:00:000'
                               ) a
  UNION ALL
  SELECT DATEADD(ms, DATEDIFF(ms,cte.a, '2013-10-24 12:00:00:000')/2, cte.a)
  FROM cte
  WHERE DATEDIFF(hour, cte.a, '2013-10-24 12:00:00:000') >= 4
)
SELECT * FROM cte;

An SQLfiddle to test with.

EDIT: To get the tasks that have a mail time that was in the last 2 hours (ie that should generate a mail), you can use something like;

WITH cte AS (
  SELECT taskid,enddate, DATEADD(s, 
                  DATEDIFF(s, startdate, enddate)/2, startdate) tm
  FROM Tasks
  UNION ALL
  SELECT taskid,enddate, DATEADD(ms, DATEDIFF(ms,cte.tm, enddate)/2, cte.tm)
  FROM cte
  WHERE DATEDIFF(hour, cte.tm, enddate) >= 4
)
SELECT taskid, tm FROM cte WHERE tm < GETDATE() AND DATEDIFF(hour, tm, GETDATE()) < 2

If the job runs at somewhat irregular intervals, you may want to truncate GETDATE() to just hours.