In response to your update - your formula is wrong. You're looking for: 
SELECT <total_time_in_system> / <number_of_times_in_system>
  FROM  . . .

[download]

So, given a table that looks something like: 

mysql> describe timestamps;
SESSION   INT
CUSTOMER  INT
TIME      TIMESTAMP
ENTRY     TINYINT
. . .
mysql>

[download]
With the constraints you could write SQL like 
SELECT SUM(UNIX_TIMESTAMP(b.time) - UNIX_TIMESTAMP(a.time))
  FROM timestamps a, timestamps b
 WHERE a.session = b.session
   AND a.entry = 1
   AND b.entry = 0
;

SELECT COUNT( * )
  FROM timestamps
 WHERE entry = 1
;

[download]

Divide the first by the second, checking to make sure that the second isn't 0. You now have the average number of seconds any given login spent on your site.

Of course, I'd make sure that the appropriate indices were there. Maybe, an index on (SESSION, ENTRY, TIME) would be appropriate.

------
We are the carpenters and bricklayers of the Information Age.

Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

I shouldn't have to say this, but any code, unless otherwise stated, is untested

In reply to Re^2: MySql Timestamps by dragonchild
in thread MySql Timestamps by Agyeya

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