Need comments on MySQL events - mysql

Question

SET GLOBAL event_scheduler = ON;
CREATE TABLE question(
qid INT AUTO_INCREMENT PRIMARY KEY,
name CHAR(30) NOT NULL,
text CHAR(100) NOT NULL,
variation BOOLEAN NOT NULL,
url CHAR(100) NOT NULL UNIQUE,
expired TIMESTAMP NOT NULL
);
CREATE TABLE alternativ(
aid INT AUTO_INCREMENT PRIMARY KEY,
name CHAR(30) NOT NULL,
text CHAR(50) NOT NULL,
number_chosen INT,
qid INT NOT NULL
);
ALTER TABLE alternativ
ADD FOREIGN KEY (qid)
REFERENCES question(qid);
CREATE EVENT delete_expired
ON SCHEDULE
EVERY 1 DAY
DO
DELETE FROM alternativ WHERE alternativ.qid IN (SELECT qid FROM question WHERE question.expired<CURRENT_TIMESTAMP)
DELETE FROM question WHERE question.expired < CURRENT_TIMESTAMP;
My question is: Should this event work with the specified database? I have tried, but it dosen't seem to work. The idea is that the database itself will delete questions that has expired. Help would much be appreciated.

Answer 1

If you want to specify multiple statements within the event body, you will need to wrap them within a compound statement block such as BEGIN ... END (in order to make such a command work, one must configure one's client to use an alternative statement delimiter in order that it does not think the first encountered semicolon terminates the CREATE EVENT statement—in the mysql command-line tool, one can use the DELIMITER command):
DELIMITER ;;
CREATE EVENT ... DO BEGIN
DELETE ... ;
DELETE ... ;
END ;;
DELIMITER ;
That said, one can delete from multiple tables with a single DELETE command using the multiple-table syntax:
DELETE alternativ, question
FROM alternativ JOIN question USING (qid)
WHERE question.expired < CURRENT_TIMESTAMP
However, all that said, you might fare better specifying foreign key constraints that cascade record deletions:
FOREIGN KEY (qid) REFERENCES question(qid) ON DELETE CASCADE
Then, one only need DELETE the referenced record (i.e. in the question table) and MySQL will delete the referencing records (i.e. in the alternativ table) for you.