SQL to determine minimum sequential days of access?

Question

The following User History table contains one record for every day a given user has accessed a website (in a 24 hour UTC period). It has many thousands of r

Answer 1

declare @startdate as datetime, @days as int
set @startdate = cast('11 Jan 2009' as datetime) -- The startdate
set @days = 5 -- The number of consecutive days

SELECT userid
      ,count(1) as [Number of Consecutive Days]
FROM UserHistory
WHERE creationdate >= @startdate
AND creationdate < dateadd(dd, @days, cast(convert(char(11), @startdate, 113)  as datetime))
GROUP BY userid
HAVING count(1) >= @days

The statement cast(convert(char(11), @startdate, 113) as datetime) removes the time part of the date so we start at midnight.

I would assume also that the creationdate and userid columns are indexed.

I just realized that this won't tell you all the users and their total consecutive days. But will tell you which users will have been visiting a set number of days from a date of your choosing.

Revised solution:

declare @days as int
set @days = 30
select t1.userid
from UserHistory t1
where (select count(1) 
       from UserHistory t3 
       where t3.userid = t1.userid
       and t3.creationdate >= DATEADD(dd, DATEDIFF(dd, 0, t1.creationdate), 0) 
       and t3.creationdate < DATEADD(dd, DATEDIFF(dd, 0, t1.creationdate) + @days, 0) 
       group by t3.userid
) >= @days
group by t1.userid

I've checked this and it will query for all users and all dates. It is based on Spencer's 1st (joke?) solution, but mine works.

Update: improved the date handling in the second solution.