Re: operating on data from multiple rows?

On Wed, 2002-10-23 at 08:06, Joshua Daniel Franklin 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?

Some alternatives that spring to mind:

(a) Use another table to reconstruct the data into a better form.

(b) Add a column for stop_time, or duration and maintain it separately
with a regular script.

(c) Write a function which returns the duration.

You could combine (b) and (c) as well.

Here's an example function that might give you ideas:

CREATE OR REPLACE FUNCTION duration_to ( DATE, TIME, INT ) RETURNS
TIMESPAN AS 'SELECT ($1::timestamp + $2) - (loggeddate::timestamp +
loggedtime) FROM log WHERE sessionid = $3 AND accntstatus = ''Start'' '
LANGUAGE 'SQL';

Then you should be able to do something like:

SELECT *, duration_to( loggeddate, loggedtime, sessionid FROM log WHERE
accntstatus = 'Stop';

Regards,
                    Andrew.

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