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@xxxxxx> 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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