From: Eric Abrahamsen <eric@ericabrahamsen.net>
To: emacs-orgmode@gnu.org
Subject: Re: Finally figuring out some ob-sqlite stuff -- for worg?
Date: Sat, 09 Nov 2019 09:58:58 -0800 [thread overview]
Message-ID: <87r22h53tp.fsf@ericabrahamsen.net> (raw)
In-Reply-To: m1zhh5ari7.fsf@nobis-it.eu
Stefan Nobis <stefan-ml@snobis.de> writes:
> Eric Abrahamsen <eric@ericabrahamsen.net> writes:
>
>> Okay, it's up. If anyone wants to explain to me the point of the
>> "where exists" clause in the SQL, I would be interested to hear. It
>> works as expected this way, but is that clause necessary?
>
> Yes, very necessary. Without it, all ratings would be changed - the
> two example rows without ratings (ids 5 and 12) would get the values
> from the intermediary org table, every other row in table bookreview
> would get its rating attribute set to null (because there is no
> matching entry in the temporary updates table).
>
> Remember: update without a where clause always touches every single
> row of the complete table.
>
> The "where exists" clause ensures that only those rows of bookreviews
> are touched that are present in the intermediary org table. If you do
> not like "where exists" you could say "where bookreview.id in (select
> id from udpates)".
Beautiful, this explains it perfectly. I had the sense that was the
purpose, but my very straightforward programming brain was insisting
that that job should be done with something like your final tip above:
update only if the id is in the updates table.
I was confused in part because the "where exists (select *..." looks
like its main purpose is to return rows. Will the select subquery
actually restrict the values that are available for updating/comparison
in the update statement? Or does the "exists" mean the subquery is
treated as a plain yes/no boolean, and the update still has access to
anything it likes? Ie, we could write "where exists (select <foo>" to
the same effect? Hope that's clear!
In essence, the "where exists" is acting as an "inner join"...
Sorry for the off-topic SQL detour! And thanks very much for this cogent
explanation.
Eric
next prev parent reply other threads:[~2019-11-09 17:59 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-08 0:10 Finally figuring out some ob-sqlite stuff -- for worg? Eric Abrahamsen
2019-11-08 6:59 ` Fraga, Eric
2019-11-09 16:50 ` Eric Abrahamsen
2019-11-09 17:28 ` Stefan Nobis
2019-11-09 17:58 ` Eric Abrahamsen [this message]
2019-11-10 9:44 ` Stefan Nobis
2019-11-10 19:48 ` Eric Abrahamsen
2019-11-10 19:48 ` Eric Abrahamsen
2020-02-12 9:15 ` Bastien
-- strict thread matches above, loose matches on Subject: below --
2019-11-08 7:06 Thomas S. Dye
2019-11-08 16:20 ` Eric Abrahamsen
2019-11-09 17:38 Thomas S. Dye
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