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

Off the top of my head, MySQLish:

SELECT start.UserId
FROM UserHistory AS start
  LEFT OUTER JOIN UserHistory AS pre_start ON pre_start.UserId=start.UserId
    AND DATE(pre_start.CreationDate)=DATE_SUB(DATE(start.CreationDate), INTERVAL 1 DAY)
  LEFT OUTER JOIN UserHistory AS subsequent ON subsequent.UserId=start.UserId
    AND DATE(subsequent.CreationDate)<=DATE_ADD(DATE(start.CreationDate), INTERVAL 30 DAY)
WHERE pre_start.Id IS NULL
GROUP BY start.Id
HAVING COUNT(subsequent.Id)=30

Untested, and almost certainly needs some conversion for MSSQL, but I think that give some ideas.