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From: Tom Lane <[email protected]>
To: Erik Wienhold <[email protected]>
Cc: Shammat <[email protected]>
Cc: [email protected]
Subject: Re: Left join syntax error
Date: Sat, 18 May 2024 11:16:58 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
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Erik Wienhold <[email protected]> writes:
> But I wonder if the implicit cross join syntax ("FROM peoples, companies")
> should actually produce this error because the explicit cross join
> works:

>     SELECT p.lname, p.fname, p.job_title, p.company_nbr, p.email, c.company_name
>     FROM people as p
>         CROSS JOIN companies as c
>         LEFT JOIN companies ON c.company_nbr = p.company_nbr;

> But I'm not even sure if implicit and explicit cross join are
> semantically equivalent.

Well, they do the same thing, but JOIN binds tighter than comma.
So in one case you have effectively

    FROM people as p CROSS JOIN
         (companies as c LEFT JOIN companies ON c.company_nbr = p.company_nbr)

and "p" is not within the scope of the JOIN/ON clause.
The other way is effectively

    FROM (people as p CROSS JOIN companies as c)
         LEFT JOIN companies ON c.company_nbr = p.company_nbr;

which is syntactically legal, although it probably doesn't do
what you wanted.

If memory serves, MySQL got this basic syntactic detail wrong
for years, as a result of which there's (still) a tremendous amount
of confusion on the net about what is the syntactic precedence in
FROM clauses.

			regards, tom lane

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