Join condition parsing puzzle
I'm looking at a query generated by SQLAlchemy. It works; Postgres is
perfectly happy to run it, and it gives answers that make sense to the
guy who wrote it. But I don't understand why it works.
Stripped way down ...
CREATE VIEW relation_a (id_c, id_v)
AS VALUES (1, 20), (2, 21), (3, 22);
CREATE VIEW relation_b (id_c, id_v, id_p)
AS VALUES (1, 20, 300), (2, 21, 301);
CREATE VIEW relation_c (id_p)
AS VALUES (301);
SELECT *
FROM relation_a
LEFT JOIN relation_b
JOIN relation_c
ON (relation_c.id_p = relation_b.id_p)
ON (relation_a.id_c = relation_b.id_c AND relation_a.id_v = relation_b.id_v);
I would have claimed before seeing this example that it wasn't even
grammatical; I thought the only legal place to write the ON clause was
immediately after the JOIN. Apparently not.
How should I read this query? I'd appreciate any help understanding this.
--
Mark Jeffcoat
Austin, TX
Mark Jeffcoat <jeffcoat@alumni.rice.edu> writes:
SELECT *
FROM relation_a
LEFT JOIN relation_b
JOIN relation_c
ON (relation_c.id_p = relation_b.id_p)
ON (relation_a.id_c = relation_b.id_c AND relation_a.id_v = relation_b.id_v);
I would have claimed before seeing this example that it wasn't even
grammatical; I thought the only legal place to write the ON clause was
immediately after the JOIN. Apparently not.
How should I read this query? I'd appreciate any help understanding this.
You read it as
SELECT *
FROM
relation_a
LEFT JOIN (relation_b
JOIN relation_c
ON (relation_c.id_p = relation_b.id_p))
ON (relation_a.id_c = relation_b.id_c AND relation_a.id_v = relation_b.id_v);
There's no other valid way to parenthesize it, so that's what
the parser does.
regards, tom lane
On Thu, Aug 23, 2018 at 4:51 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Mark Jeffcoat <jeffcoat@alumni.rice.edu> writes:
I would have claimed before seeing this example that it wasn't even
grammatical; I thought the only legal place to write the ON clause was
immediately after the JOIN. Apparently not.You read it as
SELECT *
FROM
relation_a
LEFT JOIN (relation_b
JOIN relation_c
ON (relation_c.id_p = relation_b.id_p))
ON (relation_a.id_c = relation_b.id_c AND relation_a.id_v = relation_b.id_v);There's no other valid way to parenthesize it, so that's what
the parser does.
Thank you very much for your help, Tom. In retrospect, I see I'd
over-generalized the rule that sub-selects in the from clause require
an alias.
Clear now.
--
Mark Jeffcoat
Austin, TX