MySQL trigger: if record exists just update There is a MySQL database with the following table reviews(reviewer, product, score, title, comment, creation_date, last_modified).
It is needed a trigger before insertion to check if the user has already reviewed the same product. If so, just update the fields score, title, comment and last_modified. This mean no insertion.
Otherwise no operation is needed from the trigger and the review is inserted as usually.

N.B. primary key is bold, foreign key is italic

I tried something myself but I am confident enough to say it does not work:

DELIMITER /
CREATE TRIGGER update_review BEFORE INSERT ON reviews
FOR EACH ROW
BEGIN
IF ((SELECT reviews.reviewer, reviews.product FROM reviews WHERE reviews.reviewer=new.reviewer AND reviews.product=new.product)) THEN
SET old.score=new.score AND old.title=new.title AND old.comment=new.comment AND old.last_modified=new.last_modified;
END IF;
END /
DELIMITER ;
check if the user has already reviewed the same product. If so, just update the fields score, title, comment and last_modified.


No trigger! Use INSERT .. ON DUPLICATE KEY UPDATE.

INSERT INTO reviews (reviewer, 
                     product, 
                     score, 
                     title, 
                     comment, 
                     creation_date, 
                     last_modified)
VALUES (@reviewer, 
        @product, 
        @score, 
        @title, 
        @comment, 
        @creation_date, 
        @last_modified)
ON DUPLICATE KEY UPDATE score = VALUES(score),
                        title = VALUES(title),
                        comment = VALUES(comment)
                        last_modified = VALUES(last_modified);


Additionally - define creation_date field as DEFAULT CURRENT_TIMESTAMP and last_modified field as DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, then remove them from all INSERT/UPDATE queries - they will be updated automatically.