MySQL: Update second table with trigger and IF statement - mysql

Question

I'm trying to create a trigger which sets selectoin allow to 1 when main state is updated to 3. However I can't get the query to work correctly. So far I have:
create table main
(main_id varchar(30) primary key,
name varchar(30) null,
state int null,
update_timestamp timestamp null);
create table selection
(id varchar(30) primary key,
allow varchar(30) null,
last_update_timestamp timestamp null);
//
create trigger upd_selectoin
before update on main
for each row
IF new.state = 3
THEN
UPDATE selection s
JOIN main m
ON m.main_id = s.id
SET s.allow = 1
WHERE s.id = NEW.main_id;
END IF;
END;
//
insert into main values (1,'row1',1,null);
insert into main values (2,'row2',0,null);
insert into selection values (1,null,null);
insert into selection values (2,null,null);
Error message:
Schema Creation Failed: You have an error in your SQL syntax; check
the manual that corresponds to your MySQL server version for the
right syntax to use near '//

Answer 1

You have at least two problems with the syntax:
You meant to use DELIMITER //
You forgot BEGIN
The correct definition might look like
DELIMITER //
CREATE TRIGGER upd_selectoin
BEFORE UPDATE ON main
FOR EACH ROW
BEGIN
IF NEW.state = 3 THEN
UPDATE selection s JOIN main m
ON m.main_id = s.id
SET s.allow = 1
WHERE s.id = NEW.main_id;
END IF;
END //
DELIMITER ;
Here is SQLFiddle demo
Now in your case
You probably want to use AFTER event instead of BEFORE to make sure that update in main has been made successfully before you make any updates to selection.
Also there is no point in JOINing selection with main since you already know main_id.
You can utilize INSERT ... ON DUPLICATE KEY UPDATE to make sure that a row in selection will exist even if it wasn't before
Looking at the selection schema you probably meant to assign a timestamp during that update
That being said a more succinct version of your trigger might look like
CREATE TRIGGER upd_selection
AFTER UPDATE ON main
FOR EACH ROW
INSERT INTO selection (id, allow, last_update_timestamp)
VALUES (NEW.main_id, IF(NEW.state = 3, 1, NULL), NOW())
ON DUPLICATE KEY UPDATE allow = VALUES(allow),
last_update_timestamp = VALUES(last_update_timestamp);
This version of a trigger doesn't even need DELIMITER and BEGIN ... END block because it contains only one statement.
Here is SQLFiddle demo