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Re: operating on data from multiple rows?

From: Doug Gorley <douggorley(at)shaw(dot)ca>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: operating on data from multiple rows?
Date: 2002-10-22 22:42:55
Message-ID: 1035326576.12223.4.camel@h24-69-83-179 (view raw or flat)
Thread:
Lists: pgsql-novice
On Tue, 2002-10-22 at 12:52, Michael Paesold wrote:
> Joshua Daniel Franklin <joshua(at)iocc(dot)com> wrote:
> 
> > Here is a problem I've run into with an old IMHO poorly designed database:
> >
> > There is a table ("log") that has fields
> >
> > username, sessionid, loggedtime, loggeddate, accntstatus
> >
> > A SELECT might return this data, for example:
> >
> > bob 1035208 2002-10-11 11:32:00 Start
> > bob 1035208 2002-10-11 11:38:00 Stop
> > bob 1052072 2002-10-12 10:05:00 Start
> > bob 1052072 2002-10-12 10:15:00 Stop
> >
> > I'm trying to get my head around a SELECT that will return
> > only one entry per sessionid with a duration instead of two entries for
> > each. If I had two separate tables for Start and Stop it would
> > be trivial with a join, but all I can think of is doing a
> > "SELECT ... WHERE accntstatus = 'Start'" and then grabbing the
> > sessionid and doing a separate SELECT for every record (and then the
> > math to get the duration). This seems like a bad idea since thousands
> > of records are retrived at a time.
> > Am I missing a better way?
> 
> A self-join would help...
> 
> SELECT start.username, start.sessionid,
>     ((stop.loggeddate + stop.loggedtime)
>         - (start.loggeddate + start.loggedtime)) as duration
>   FROM log AS start, log AS stop
>   WHERE start.accntstatus = 'Start'
>     AND stop.accntstatus = 'Stop'
>     AND start.sessionid = stop.sessionid;
> 
> (not tested, but try like this)
> You probably have to cast the value of the duration.
> 
> Best Regards,
> Michael Paesold
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster

I second this idea; in fact, given the difficulty in getting useful
information from the current table, you might even want to consider
building a view:

-----------------------

create view log_info as
  select l1.username,
         l1.sessionid,
         l1.loggeddate + l1.loggedtime as start,
         l2.loggeddate + l2.loggedtime as stop,
         case
           when l2.sessionid is not null then
             (
               ( l2.loggeddate + l2.loggedtime )
               - ( l1.loggeddate + l1.loggedtime )
             )
           else
             date_trunc( 'second',
                         now() - ( l1.loggeddate + l1.loggedtime ) )
         end as duration
  from ( select * from log where accntstatus = 'Start' ) l1
    left outer join ( select * from log where accntstatus = 'Stop' ) l2
    on ( l1.sessionid = l2.sessionid )
;

-- 
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