Microsoft SQL server count distinct every 30 minutes - sql

Question

We have an activity database that records user interaction to a website, storing a log that includes values such as [UserId] and [LogDate] e.g.
UserId|LogDate
123 |2017-01-01 11:17:35.190
I am trying to find out the count of distinct user sessions over time.
This would be easy enough by counting the distinct users:
SELECT COUNT(DISTINCT UserId) FROM ActivityDatabase.dbo.Logs
However, I need to count a user multiple times if they have a log more than 30 minutes from the previous log as this is then classed as a new session.
A session is defined as having a log in a 30 minute timeframe. For example:
If a user creates a log at 13.30, the value for distinct user
sessions over time would be 1.
If the user creates another log at 13.40, the count should still be 1 as it is within 30 minutes of the previous log.
If the user creates another log at 14.20, the count should then be 2 as this is 30 minutes after the previous log.
Is this possible in SQL? I would need a way of checking every log for a user against the previous user log, and if the time difference between these is more than 30 minutes, it should count as a unique session.
The output of the SQL should be a number rather than broken down by a time period.
Thank you.

Answer 1

Sessionizing is a bit tricky. Let me show you how to do that. Perhaps this will solve your problem:
select userid, min(log_date) as session_start,
dateadd(minute, 30, max(log_date)) as session_end,
row_number() over () as session_id
from (select l.*,
sum(case when log_date < dateadd(minute, 30, prev_logdate)
then 0 else 1
end) over (partition by userid order by logdate
) as grp
from (select l.*,
lag(logdate) over (partition by userid order by logdate) as prev_logdate
from ActivityDatabase.dbo.Logs l
) l
) l
group by userid, grp;
If you want the number of unique users at a given point in time, then:
with s as (
select userid, min(log_date) as session_start,
dateadd(minute, 30, max(log_date) as session_end,
row_number() over () as session_id
from (select l.*,
sum(case when log_date < dateadd(minute, 30, prev_logdate)
then 0 else 1
end) over (partition by userid order by logdate
) as grp
from (select l.*,
lag(logdate) over (partition by userid order by logdate) as prev_logdate
from ActivityDatabase.dbo.Logs l
) l
) l
group by userid, grp
)
select count(*)
from s
where #datetime between session_start and session_end;
A more brute force alternative for a given time is:
select count(distinct userid)
from ActivityDatabase.dbo.Logs l
where #datetime between log_date and dateadd(minute, 30, log_date);

Answer 2

If you are using sql server 2012 or greater, I would use the lag function to find the previous row and then you can compare the two datetimes to see if the difference is greater than 30 mins
select
userId,
LogDate,
LAG(LogDate, 1,0) OVER (PARTITION BY userId ORDER BY LogDate) AS PreviousLogDate
from logTbl
You can then add datediff and a case statement to flag a new login where the difference is greater than your threshold.
If no previous row is found, then the lag function will return null.

Answer 3

If you play around with the definition you're trying to use, it becomes a lot easier to write the SQL.
What we want to identify are "starting logs" - logs that mark the start of a session. We don't want to identify any other logs.
How do we define a "starting log"? It's a log that doesn't have another log within 30 minutes before it.
SELECT COUNT(*)
FROM ActivityDatabase.dbo.Logs l1
WHERE NOT EXISTS (
SELECT * FROM ActivityDatabase.dbo.Logs l2
WHERE l1.UserId = l2.UserId AND
l2.LogDate < l1.LogDate AND
l2.LogDate >= DATEADD(minute,-30,l1.LogDate)
)