oracle.sql

CREATE OR REPLACE TRIGGER BidHighEnough
BEFORE INSERT ON Bids
REFERENCING NEW AS new
FOR EACH ROW
DECLARE numHigherBids INT;
BEGIN
    SELECT COUNT(amount) INTO numHigherBids
    FROM   Bids
    WHERE  item = :new.item AND amount > :new.amount;

    IF ( numHigherBids > 0 ) THEN
        RAISE_APPLICATION_ERROR(-20000, 'bid not high enough');
    END IF;
END;

postgresql.sql

CREATE OR REPLACE FUNCTION bid_high_enough() RETURNS TRIGGER AS $_$
BEGIN
  IF EXISTS (SELECT * FROM bids WHERE item = NEW.item AND amount > NEW.amount) THEN
    RAISE EXCEPTION 'Bid not high enough';
  END IF;
  RETURN NEW;
END $_$ LANGUAGE 'plpgsql';

DROP TRIGGER IF EXISTS BidHighEnough ON bids;

CREATE TRIGGER BidHighEnough BEFORE INSERT ON Bids
FOR EACH ROW
EXECUTE PROCEDURE bid_high_enough();