Erik Wienhold <e...@ewie.name> 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
