Re: Left join syntax error

On Saturday, May 18, 2024, Erik Wienhold <ewie(at)ewie(dot)name> wrote:

> On 2024-05-18 15:19 +0200, Shammat wrote:
> > Am 18.05.24 um 14:52 schrieb Rich Shepard:
> > > It's been a _very_ long time since I wrote a SQL script and, despite
> looking
> > > at my SQL books and web pages, I don't know how to fix the error.
> > >
> > > The three line script is:
> > > -----
> > > SELECT p.lname, p.fname, p.job_title, p.company_nbr, p.email,
> c.company_name
> > > FROM people as p, companies as c
> > > LEFT JOIN companies ON c.company_nbr = p.company_nbr;
> > > -----
> > >
> > > and psql responds:
> > > ERROR: invalid reference to FROM-clause entry for table "p"
> > > LINE 3: LEFT JOIN companies ON c.company_nbr = p.company_nbr;
> > > ^
> > > HINT: There is an entry for table "p", but it cannot be referenced
> from this part of the query.
> >
> > Don't put the second table in the FROM part
> >
> > SELECT p.lname, p.fname, p.job_title, p.company_nbr, p.email,
> c.company_name
> > FROM people as p
> > LEFT JOIN companies as c ON c.company_nbr = p.company_nbr
>
> Yes, Rich probably just wants the left join.
>
> 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. The docs on FROM [1] sort of imply that:
>

Too lazy to find the docs right now but what you are observing is basically
an operator precedence effect. The comma join hasn’t happened at the time
the left join is evaluated and so other tables in the comma join cannot
appear in the on clause of the left join. Placing everything inside a
single from slot and moving the conditions to the where clause removes
changes the precedence aspect so that the cross join does indeed evaluate
prior to the left join.

I’m content with not pointing out this possible gotcha in the documentation.

David J.