From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | "Karl O(dot) Pinc" <kop(at)meme(dot)com> |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Binding of "AS" vis "JOIN" |
Date: | 2005-02-04 18:15:03 |
Message-ID: | 15911.1107540903@sss.pgh.pa.us |
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Lists: | pgsql-general |
"Karl O. Pinc" <kop(at)meme(dot)com> writes:
> I'm doing some complicated joining and am getting error
> messages about unknown relations and can't figure out
> what's up. I'm wondering if "as" aliasing gives
> an alias to the product of a join, not just the
> one table that appears immediately in front of the
> "as". ?
If you write
... FROM (a JOIN b) AS c
then (1) the alias c refers to the whole join result, not either a or b
and (2) you can't directly reference the table aliases a or b from
outside this JOIN clause: the alias c masks them. This behavior is per
SQL spec.
> The other question that sprang to mind while working
> on this was whether there's performance (or any other)
> implications of using WHERE conditions instead of
> JOIN inside FROM.
See
http://www.postgresql.org/docs/8.0/static/explicit-joins.html
(adjust URL to match the major version you are running, as the answers
vary).
regards, tom lane
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