Generate Lab Statistics From Data

Hello,

I'm a network admin who understands sql but have no reason to write it everyday so I have no idea how to approach this problem.

I am trying to generate usage statistics of our computer labs. I have written a few scripts that populate a sql 2005 database with the following information:

[datetime] [username] [computer] [event]
2007-10-10 15:25:03 tom earth logon
2007-10-10 15:39:09 john mars logon
2007-10-10 15:41:13 dave pluto logoff
2007-10-10 15:47:29 john mars logoff
2007-10-10 15:59:48 tom earth logoff

Each time a user logs on or logs off one of our lab computers the information is inserted into this database.

It seems that was the easy part. Lacking any local sql experts, I'm pondering where to go from here to generate usage statistics.

What I'm thinking is a SQL query (which I don't know how I'd write) that would calculate the session time and insert it into a new table. A session is defined as the time between a logon and logoff where the username and computer match.

The new table would look something like this:

[user] | [computer] | [sessionTime] | [day]
tom | earth | 00:34:45 | 2007-10-10
john | mars | 00:08:20 | 2007-10-10

The logic of the query finds the first logon event and matches it with the first logoff event in which the username and computer match. It then subtracts the logon datetime from the logoff to calculate session time.

Is this a reasonable approach? Is the query easy to write?