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To: newsy@lewczuk.com
Cc: "'Karsten Hilbert'" <Karsten.Hilbert@gmx.net>,
	"'Lista dyskusyjna pgsql-general'" <pgsql-general@postgresql.org>,
	"Lista dyskusyjna pgsql-sql" <pgsql-sql@postgresql.org>
Subject: Re: [SQL] SQL query problem (many table in FROM statement and many
	LEFT JOIN's)
In-reply-to: <004001c38f03$8d8b8230$4701a8c0@marek> 
References: <004001c38f03$8d8b8230$4701a8c0@marek>
Comments: In-reply-to "Marek Lewczuk" <newsy@lewczuk.com>
	message dated "Fri, 10 Oct 2003 08:53:11 +0100"
Date: Tue, 14 Oct 2003 10:35:06 -0400
Message-ID: <9190.1066142106@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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"Marek Lewczuk" <newsy@lewczuk.com> writes:
> ... It also working fine. !!!! The question is, why my first query isn't
> working:

> SELECT
>   _CON.con_id,
>   _MOD.mod_ty,
>   _VER.version,
>   _YEA.year,
>   _CON.dri_id,
>   _CON.man_cod,
>   _ENG.eng_pow
> FROM
>   db_data.mda_mod _MOD,
>   db_data.mda_mod_con _CON,
>   db_data.mda_mak_eng _ENG,
>   db_data.set_mda_fue _FUE
>   LEFT JOIN db_data.mda_mod_ver _VER ON _VER.ver_id = _CON.ver_id
>   LEFT JOIN db_data.mda_mod_yea _YEA ON _YEA.yea_id = _CON.yea_id 
> WHERE
>   _MOD.mod_id = '283' AND
>   _CON.mod_id = _MOD.mod_id AND
>   _CON.psd <= NOW() AND
>   _CON.ped > NOW() AND
>   _ENG.eng_id = _CON.eng_id AND
>   _ENG.eng_fue = _FUE.fue_id

The reason that works in MySQL and fails in Postgres is that MySQL isn't
compliant with the SQL standard.  The standard says that the above FROM
clause means that _FUE is left-joined to _VER, then that result is
left-joined to _YEA, then the _MOD, _CON, and _ENG tables are joined to
that result (in no particular order).  You get the error because the
LEFT JOIN ON clauses refer to _CON which is not part of what they are
joining.

I believe that MySQL interprets the above statement as "join the tables
in the order listed in the FROM clause", that is they join
_MOD/_CON/_ENG/_FUE, then left-join _VER to that result, etc.  This is a
hangover from days when they didn't actually have a query planner.
Unfortunately, it's not SQL, it's only something that looks like SQL.

			regards, tom lane