Trigger Syntax Error in MySQL on IF/THEN (ERROR 1064) - mysql

Question

I have an error while executing this query:
CREATE TRIGGER insert_Topics
BEFORE INSERT
ON Topics
FOR EACH ROW
BEGIN
IF (SELECT COUNT(*) FROM Subjects WHERE ID=new.SubjectID)=0
THEN
INSERT error_msg VALUES ('Foreign Key Constraint Violated!');
END IF;
END;
delimiter ;
The error says:
1064 - 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 '' at line 8
and it points to the word THEN
Any help? Thanks in advance.

Answer 1

Both
CREATE TRIGGER insert_Topics
BEFORE INSERT
ON Topics
FOR EACH ROW
BEGIN
IF NOT EXISTS(SELECT 1 FROM Subjects WHERE ID=NEW.SubjectID LIMIT 1) THEN
INSERT INTO error_msg VALUES ('Foreign Key Constraint Violated!');
END IF;
END;
and
CREATE TRIGGER insert_Topics
BEFORE INSERT
ON Topics
FOR EACH ROW
BEGIN
IF (SELECT 1 FROM Subjects WHERE ID=NEW.SubjectID LIMIT 1) IS NULL THEN
INSERT INTO error_msg VALUES ('Foreign Key Constraint Violated!');
END IF;
END;
work for me, so your syntax problems probably lie elsewhere.

Answer 2

I know this is an old question, but I thought I'd share my thoughts over what I think the error was, in case anyone sees this in the future (feel free to community-edit if i've made any mistakes!).
The reason there were issues in the original question seems to be with the use of DELIMITER, and what it does, as explained here: What does DELIMITER // do in a Trigger?
In the given code:
INSERT error_msg VALUES ('Foreign Key Constraint Violated!');
END IF;
END;
delimiter ;
The resetting of delimiter with "delimiter ;" indicates that it changed previously to another delimiter, say:
DELIMITER //
--TRIGGER1
--TRIGGER2
--TRIGGERn
delimiter ; -- Aggravates me that this wasn't capitalised :P
The reason for the delimiter to be set to "//" (or others), is to allow multiple statements to be delimited, and what I'm thinking, is that the original author had other triggers above that trigger, and had changed the delimiter to something like "//", but had forgotten to change:
END IF;
END;
delimiter ;
To:
END IF;
END//
delimiter ;
Therefore, the reason why commenters weren't able to replicate, was because they had the correct delimiter set (";"), and as there was only one statement to delimit, didn't find any errors, as the author did.
See the following test code and results, including initialisation, as some proof:
DELIMITER ; -- Just in case
DROP TABLE IF EXISTS Topics, Subjects, error_msg;
CREATE TABLE Subjects ( id INT(5) PRIMARY KEY,
subjectname CHAR(20));
CREATE TABLE Topics ( topicname CHAR(20),
subjectID INT(5)
# FOREIGN KEY (subjectID) REFERENCES Subjects(id) - How this check should be done...
);
CREATE TABLE error_msg ( Message VARCHAR(50) );
INSERT INTO Subjects VALUES
('5', 'Arts'),
('55', 'Maths'),
('2342', 'Biology'),
('12345', 'Finance');
DELIMITER $$ -- Where the delimiter would be defined, originally
CREATE TRIGGER InsertOnTopics
BEFORE INSERT ON Topics
FOR EACH ROW
BEGIN
IF (SELECT COUNT(*) FROM Subjects WHERE id=NEW.subjectID)=0 THEN
INSERT error_msg VALUES ('Foreign Key Constraint Violated!');
END IF;
END$$
DELIMITER ;
INSERT INTO Topics VALUES
('Welfare Benefits', '5'),
('Eigenvectors', '55'),
('Mitochondria', '2342'),
('Bank of Dad', '12345'),
('Something else', '555');
SELECT * FROM error_msg;
With "end;", as the original code:
mysql> SOURCE /Users/benpalmer/test.sql
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.02 sec)
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.02 sec)
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
Query OK, 0 rows affected (0.02 sec)
ERROR 1064 (42000): 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 '('Something else', '555')' at line 6
Empty set (0.00 sec)
mysql>
With "END$$", in my code:
mysql> SOURCE /Users/myUsername/test.sql
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.02 sec)
Query OK, 0 rows affected (0.02 sec)
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
Query OK, 0 rows affected (0.02 sec)
Query OK, 5 rows affected (0.01 sec)
Records: 5 Duplicates: 0 Warnings: 0
+----------------------------------+
| Message |
+----------------------------------+
| Foreign Key Constraint Violated! |
+----------------------------------+
1 row in set (0.00 sec)
mysql>
Other than this, I don't understand why this particular example isn't being done with a proper foreign key constraint. As my first answer... Hope this helps anyone! Was also stuck on this for a while.