On Saturday, May 18, 2024, Erik Wienhold <
ewie@ewie.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.
