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?
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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