On May 29, 2007, at 9:48 , Robert James wrote:
I'd like to be able to detect if a record has associations. I don't
want to actually delete it, just know if it could be deleted. (This
is to build an intelligent menu on a GUI)
On 5/29/07, Albe Laurenz <[EMAIL PROTECTED]> wrote: > Is there
anyway to know if a DELETE will violate an FK
> without actually trying it?
I don't know what you mean by 'without trying it', but does the
following answer your question?
CREATE TABLE a (id integer PRIMARY KEY);
CREATE TABLE b (id integer PRIMARY KEY,
a_id integer NOT NULL CONSTRAINT b_fkey REFERENCES a(id));
INSERT INTO a (id) VALUES (1);
INSERT INTO b (id, a_id) VALUES (42, 1);
[Please don't top post. It makes referencing the previous text more
difficult.]
It sounds like you can find what you want just by using JOIN or one
of the workarounds for SEMIJOIN. Continuing with Albe's example:
INSERT INTO a (id) VALUES (2), (3), (4), (5);
INSERT INTO b(id, a_id) VALUES (43, 3), (45, 5);
Records that match (using JOIN):
SELECT *
FROM (
SELECT id as a_id
FROM a) AS a
NATURAL JOIN b;
a_id | id
------+----
1 | 42
3 | 43
5 | 45
(3 rows)
Records that don't match (SEMIJOIN workaround using EXCEPT)
SELECT *
FROM (
SELECT id as a_id
FROM a) AS a
EXCEPT
SELECT a_id
FROM b;
a_id | id
------+----
2 |
4 |
(2 rows)
Records that don't match (SEMIJOIN workaround using LEFT JOIN)
SELECT *
FROM (
SELECT id AS a_id
FROM a) AS a
LEFT JOIN b USING (a_id)
WHERE id IS NULL;
a_id | id
------+----
2 |
4 |
(2 rows)
Both records that match and don't match using LEFT JOIN:
SELECT *
, CASE WHEN id IS NULL
THEN FALSE
ELSE TRUE
END AS has_record
FROM (
SELECT id AS a_id
FROM a) AS a
LEFT JOIN b USING (a_id);
a_id | id | has_record
------+----+------------
1 | 42 | t
2 | | f
3 | 43 | t
4 | | f
5 | 45 | t
(5 rows)
Note you don't need the has_record column, but it might be helpful to
pass that to your application.
Hope this helps.
Michael Glaesemann
grzm seespotcode net
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