My understanding from all literature at my disposal was that table aliases
can be bestowed upon either literal table names or on subqueries.
I have now noticed that if you parenthesise a join,
as in the following example:
------------------------------------------------------
CREATE TABLE foo (id, wkd, a, b, c);
INSERT INTO foo VALUES ('Yes', 'Mon', 1, 2, 3),
('No', 'Tue', 2, 3, 5),
('Maybe', 'Wed', 3, 4, 7);
CREATE TABLE bar (id, a, b, c);
INSERT INTO bar VALUES ('Yes', 1, 2, 4),
('No', 2, 3, 5),
('Maybe', 3, 6, 7);
UPDATE foo AS f
SET a = ba.a,
c = ba.c,
wkd = ba.wkd
FROM
(bar JOIN (SELECT 'Thu' AS wkd)) AS ba
WHERE f.id = ba.id;
------------------------------------------------------
the table alias works perfectly (both on wkd and on the id column).
It also seems possible to assign an alias to the result of a set operation,
such as UNION (Example uses table "foo" from above):
------------------------------------------------------------
SELECT take_two.a FROM
(SELECT * FROM foo UNION ALL SELECT * FROM foo) AS take_two;
------------------------------------------------------------
also works.
I am now wondering if I am just too stupid to follow obvious logic or
whether this is a feature that's merely undocumented (or the documentation
of which I haven't been able to find), or some behaviour that just came
about unintentionally.
Whatever the cause, the behaviour seems useful to me, but since one should
never rely on undocumented features, I would like to inquire:
- is this intentional?
- is it realiably supported?
- is it official?
If so, I would appreciate very much finding it in the documentation. I WOULD
like to use it, but am weary of writing code that may break at some point in
the future.
Many thanks in advance!
Florian