2009/2/4 Baron Schwartz <ba...@xaprb.com> > On Tue, Feb 3, 2009 at 12:24 PM, Jerry Schwartz > <jschwa...@the-infoshop.com> 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/ > Our Services: http://www.percona.com/services.html > > -- > MySQL General Mailing List > For list archives: http://lists.mysql.com/mysql > To unsubscribe: > http://lists.mysql.com/mysql?unsub=claudio.na...@gmail.com > >
