On 4/19/07, Markus Holzer <holli.holzer@googlemail.com> wrote:
> Hello.
>
> I'm currently developing my first web app with Postgres and I have a
> question.
>
> How do I perform a conditional insert/update?
>
> To clarify: I need to insert data into a table when the primary key is not
> already in the table, or an update if it is. I have currently solved this by
> SELECTing for the primary key, then looking if there is a row, and if there
> is I do an UPDATE otherwise I do an INSERT. But since this is a web app this
> way of course leaves a big race condition.
>
> Is there a way to get around that? Like the ON DUPLICATE KEY UPDATE
> statement in MySQL?
>
>
> Thanks for your time,
>
> Holli
--------------------------------------------------
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN LOOP UPDATE db SET b = data WHERE a = key; IF found THEN RETURN; END IF;
BEGIN INSERT INTO db(a,b) VALUES (key, data); RETURN; EXCEPTION WHEN unique_violation
THEN -- do nothing END; END LOOP;
END;
$$
LANGUAGE plpgsql;
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
--------------------------------------------------
Straight from the docs:
http://www.postgresql.org/docs/8.2/static/plpgsql-control-structures.html#PLPGSQL-ERROR-TRAPPING
