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