The request has no destination for the result data after starting

Question

I have a problem with my trigger. When inserting a new line, it checks to see if the article has been sold. I can do this in software, but I think it is better when DB does it.

-- Create function
CREATE OR REPLACE FUNCTION checkSold() RETURNS TRIGGER AS $checkSold$
    BEGIN
        SELECT offer_id FROM offer WHERE offer_id = NEW.offer_id AND date_sale IS NULL;

    IF NOT FOUND THEN
        RAISE EXCEPTION 'The Offer is Sold!';
    END IF;
    RETURN NEW;
END;
$checkSold$ LANGUAGE plpgsql;


-- Create trigger
Drop TRIGGER checkSold ON tag_map;
CREATE TRIGGER checkSold BEFORE INSERT ON tag_map FOR EACH ROW EXECUTE PROCEDURE checkSold();

INSERT INTO tag_map (tag_id,offer_id) VALUES (824,80);

And this is an error after insertion.

[WARNING  ] INSERT INTO tag_map (tag_id,offer_id) VALUES (824,80)
            ERROR:  query has no destination for result data
            HINT:  If you want to discard the results of a SELECT, use PERFORM instead.
            CONTEXT:  PL/pgSQL function "checksold" line 2 at SQL statement

What happened to the trigger? It works great without it.

Answer 1

Replace

SELECT offer_id FROM offer WHERE offer_id = NEW.offer_id AND date_sale IS NULL;

from

PERFORM offer_id FROM offer WHERE offer_id = NEW.offer_id AND date_sale IS NULL;

as suggested.

Additional information in the manual ( "38.5.2 Executing a command without result" ).