Re: SELECT question

[Michael Glaesemann <grzm@xxxxxxxxxxxxxxx>]

On Aug 17, 2007, at 7:27 , Michelle Konzack wrote:

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> I have a very huge table of references from Websites (html, pics, ...)
> where the elements stored on a fileserver using sha384.
>
> Now the indextable hold all filenames and download dates but now I  
> like
> to get a "snapshoot on a paticular day.
>
> How must I create the SELCT statement to get ALL files valid on a
> particular day?
>
> Note:  There can be every time a new index.html for example but images
>        change only once a month...
>
>        So I need all elements valable on the paticular day which mean,
>        I need to select that LAST version of the elements...

I think what you want is something like:

SELECT DISTINCT ON (website_reference) website_reference,  
download_date, file_path
FROM indextable
WHERE download_date <= ? -- whatever date you're interested in
ORDER BY website_reference, download_date DESC;

This should return the most recent website_reference and its  
download_date that's earlier than the download_date specified in the  
WHERE clause.

DISTINCT ON is a (very helpful) PostgreSQL extension. You can get  
similar results using a subquery;

SELECT website_reference, download_date, file_path
FROM indextable
NATURAL JOIN (
    SELECT website_reference, max(download_date) as download_date
    FROM indextable
    WHERE download_date <= ?
    GROUP BY website_reference
    ) most_recent_versions;

This may return more than one row per website_reference if the  
website_reference has more than on file_path for a particular  
download_date.

Does this help? If not, could you give a bit more of a concrete example?

(Is is just me or have there been a lot of queries that can be solved  
using DISTINCT ON recently?)

Michael Glaesemann
grzm seespotcode net

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