2009/2/4 Baron Schwartz <baron@stripped>
> On Tue, Feb 3, 2009 at 12:24 PM, Jerry Schwartz
> <jschwartz@stripped> wrote:
> > Somebody, I think it was somebody from MySQL, said that you should never
> put
> > anything into a WHERE clause that could be put into the ON clause of a
> JOIN.
> > My guess is that this helps with the optimization, but it seems
> > counter-intuitive to me. I've never followed that advice, but I'm
> starting
>
> There are two things here -- one is coding style. If you're using all
> [INNER] JOIN then it doesn't matter, they are equivalent and the
> optimizer is smart enough to know that a condition holding two columns
> equal is the same whether it's in the WHERE or the ON. Take a look at
> the output of EXPLAIN EXTENDED... SHOW WARNINGS to see how the
> optimizer rewrites the query.
>
> There is no way to write a LEFT JOIN without putting the criteria into
> the ON clause however.
>
> As a matter of style I do not like comma-joins with the criteria in
> the WHERE clause. I want ON clauses to show me how two tables are
> related, and the WHERE clause to show me how the results are filtered
> thereafter.
HOLY WORDS!
LOGIC MATTERS!!!!
DEFINITELY: The ON clause is for joining tables, WHERE is for filtering.
If we lose the logics and set theory behind SQL we lose the way.
I really dont understand what this is supposed to mean:
----------------------------------------------------------------------------------------------
SELECT prod.prod_num, prod.prod_title, prod.prod_samp_doc
-> FROM pub JOIN prod ON pub.pub_id = prod.pub_id
-> AND pub.pub_code = 'dc'
-> WHERE prod.prod_discont = 0
-> AND prod.prod_samp_doc IS NOT NULL
-> ORDER BY prod.prod_num\G
----------------------------------------------------------------------------------------------
I am quite surprised that MySQL allows to put a condition in the JOIN clause
that can refer to a constant value
Cheers
Claudio
But oddly enough some people think this is horrible and
> they think it's much clearer to do it the reverse way! Everyone's
> entitled to their own wrong opinion ;-) Seriously it is just a matter
> of style until you get to LEFT JOIN.
>
> --
> Baron Schwartz, Director of Consulting, Percona Inc.
> Our Blog: http://www.mysqlperformanceblog.com/
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