Using triggers to auto-update aggregates with MYSQL

DROP TRIGGER IF EXISTS `db_name`.`ins_ratings_update`//
CREATE TRIGGER `db_name`.`ins_ratings_update` AFTER INSERT ON `db_name`.`ratings`
FOR EACH ROW BEGIN
UPDATE aggregate_storing_table ast SET rating = (SELECT AVG(score) FROM individual_rating_record_table WHERE ast_id=NEW.ast_id);
END
//