troubleshooting Question

# Calculate Average Time Spent on a site via SQL ?

Microsoft SQL Server
I wrote a script to track my website visitors using a really simple method:

TABLE: Visitors
Column: URL (varchar)
Column: Date/Time (DateTime)

Using this I can count how many visits/unique vistors, etc to a specific page or to the entire site. within a date range.  I can also calculate average page views per visitor and other basic web stats.

What I would like to figure out now is how to view the "average visit length" - or the average amount of time each user spent on the site before they went away.  Obviously this won't be 100% accurate, since they could spend 20 minutes on the LAST page they looked at and then just close their browser, but I can get a pretty good idea.

I'm wondering if there is a way to do this through SQL with the data present.  I would basically need to do 3 things:

(1) Count the number of unique visitors (ie; unique numbers in the IPAddress field)

(2) Get the amount of time spent on the site for each visitor.  Sometimes visitors will come to the site 2 or 3 times a day so I'm not sure exactly how to come up with this.  My first thought was to take the DateDiff from Max(Datetime) and Min(datetime) but if the user comes on and 8:00 AM and then again at 8:00 PM this method would show they were on the site for 12 hours, which we know isn't realistic.  Not really sure what to do here - maybe base it on a 15 minute window, where if a particular visitor's hits are more than 15 minutes apart it doesn't count that last 15 minutes?

In other words... someone has hits at 8:00 AM, 8:05 AM, 8:10 AM, 9:30 AM, 9:35 AM ... we will assume that they were connected from 8:00 AM to 8:10 AM simultaneously since those are close together, but their last two visits at 9:30 AM and 9:35 AM were because they opened their browser later and came back.  So THAT one person's "average visit length" would be 7.5 minutes, NOT 1 hour and 35 minutes.

(I came up with 7.5 by assuming two sessions, one from 8:00 to 8:10 for 10 minutes, and one from 9:30 to 9:35 for 5 minutes.  15 Minutes total, two sessions, = average of 7.5 minutes)

Then of course this leads to the third step ... which is to take the total SUM of the average session for each visitor and divide that by the total number of visitors to create an "Average visit length" for the site.

The question is can this be done through a stored procedure that wouldn't kill the server every time it was run?  I don't even know where to begin coding something like this.
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