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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