Mysql syntax error with trigger - mysql

Question

I have this trigger
CREATE TRIGGER checkcollision AFTER UPDATE ON players BEGIN
SELECT RAISE(ABORT, 'collision') FROM walls WHERE NEW.x=x AND NEW.y=y;
END;
mysql 5.1.72-0ubuntu0.10.04.1 (Ubuntu)
But I am getting a syntax error, and I don't see where...
EDIT:
DELIMITER //
CREATE TRIGGER checkcollision AFTER UPDATE ON players BEGIN SELECT RAISE(ABORT, 'collision') FROM walls WHERE NEW.x=x AND NEW.y=y; END//
DELIMITER ;
This is still getting a syntax error...
This is getting a syntax error:
DELIMITER //
CREATE TRIGGER checkcollision AFTER UPDATE ON players
FOR EACH ROW
BEGIN
IF (SELECT count(*) FROM walls WHERE NEW.x=x AND NEW.y=y)>0 THEN
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Collision detected';
END IF;
END;//
DELIMITER ;

Answer 1

You probably didn't change DELIMITER
How do I do that?
With the DELIMITER command, described here
http://dev.mysql.com/doc/refman/5.6/en/mysql-commands.html
http://dev.mysql.com/doc/refman/5.6/en/stored-programs-defining.html
It's important to change the delimiter when you're creating triggers or procedures, because otherwise using the semicolon statement delimiter inside the body of your trigger (e.g. at the end of the SELECT statement) is ambiguous with respect to the semicolon at the end of the CREATE TRIGGER statement.
This is a very common source of confusion for MySQL developers.

Answer 2

Edit 1:
How do I get it to roll back the change if there is a collision then
Rollback, commit are not allowed in triggers.
Instead, you can raise a signal by setting specific sqlstate on a condition failure.
IF ( condition_for_collision_true ) THEN
SET error_message = 'Invalid XYZ'; -- set proper message
-- set proper error state number
SIGNAL SQLSTATE <ERROR_STATE_NUMBER> SET message_text = error_message;
END IF;
This causes the transaction be aborted.
Original answer:
Triggers get fired for each row affected.
And you are missing the same in your trigger definition.
DELIMITER //
CREATE TRIGGER checkcollision
AFTER UPDATE ON players
FOR EACH ROW
BEGIN
SELECT RAISE(ABORT, 'collision') FROM walls WHERE NEW.x=x AND NEW.y=y;
END;
//
DELIMITER ;
And, it is obvious. Triggers are not regular routines. They are for background action. You can't expect them to return a cursor or any other result. But perform an action like setting or resetting a value in a row or any DML operation on other related table, etc.
Change the body accordingly.