Re: DISTINCT to get distinct *substrings*?

[Christoph Pingel <ch.pingel@xxxxxx>]

Thanks for the input, I think I get this now. In my case, the query

SELECT DISTINCT ON (substring(attribute from '^http://[^/]*/')) attribute from pg_atp where attribute like 'http://%' 

doesn't get me just the root of the URL, but the whole URL - but only for the first row for each individual root. While 

SELECT DISTINCT substring(attribute from '^http://[^/]*/') from pg_atp where attribute like 'http://%' 

does what I first intended - get a list of all (distinct) root URLs. Wieder was gelernt. (Learnt something again. :-)

best regards,

Christoph

Am 08.08.2006 um 20:36 schrieb Ben:

DISTINCT ON is extremely useful when you know what you're doing. It's postgres' version of oracle's first_value analytical function, and when you need it, nothing else really suffices.

On Tue, 8 Aug 2006, Nikolay Samokhvalov wrote:

SELECT DISTINCT substring(attribute from '^http://[^/]*/') from pg_atp

where attribute like 'http://%';

w/o DISTINCT there should be duplicates (if any)

don't use "DISTINCT ON" at all, it's evil :-) (why?

http://chernowiki.ru/index.php?node=38#A13)

On 8/8/06, Christoph Pingel <ch.pingel@web.de> wrote:

Hello to the list,

here's an SQL question, I hope it's not off topic. From a list of URLs I

want to get only the distinct values of the *web sites* these URLs belong

to, that is everything before and including the 3rd slash, and I think this

should be possible within the DB. I would like to say something like

SELECT substring(attribute from '^http://[^/]*/') from pg_atp where

attribute like 'http://%'

(which works) but get only the distinct values. SELECT DISTINCT ON

substring.. doesn't work. Probably I haven't understood the semantics of the

DISTINCT keyword. Can anybody help?

thanks in advance

Christoph

-- 

Best regards,

Nikolay

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