Re: how to calculate differences of timestamps?

On 09/26/2011 06:31 PM, Andreas wrote:
> How could I calculate differences of timestamps in a log-table?
>
> Table log ( user_id integer, login boolean, ts timestamp )
>
> So login = true would be a login-event and login = false a logout.
> Is there a way to find the matching login/logout to calculate the 
> difference?
>
> Or is there a better table "design" to do this?
>

One way is a sub_select:

select    o.user_id,    o.ts as logout_time,    (select         max(i.ts)     from         log i     where
i.user_id= o.user_id and         i.ts < o.ts and         login    ) as login_time
 
from    log
where    not login
;

This will give you login/logout time pairs. Just replace the "," with a 
"-" if you are interested in login duration.

Depending on the frequency and duration of logins and the number of 
users you may have to play with indexes though an index on ts will 
probably suffice for most cases.

Cheers,
Steve