CREATE TRIGGER test BEFORE INSERT ON MYTABLE
FOR EACH ROW
BEGIN
IF break < 0 THEN
SIGNAL sqlstate '45000'
set message_text = 'ERROR';
END IF;
END;
This throws a syntax error and I'm not able to find it.
You need to change the delimiter that MySQL is using as an end-of-line marker. You need to do this for CREATE TRIGGER and CREATE PROCEDURE because if you don't MySQL will treat the first semi-colon as the end of the command and finish processing it prematurely - hence the syntax error.
Do this:
DELIMITER $$
CREATE TRIGGER test BEFORE INSERT ON MYTABLE
FOR EACH ROW
BEGIN
IF break < 0 THEN
SIGNAL sqlstate '45000'
set message_text = 'ERROR';
END IF;
END$$
DELIMITER ;
Now, MySQL will continue processing your CREATE TRIGGER command until it reaches $$, and include the semi-colons as part of the trigger script. Note that I've set the delimiter back to semicolon when I'm finished.
Related
I am trying to put a constraint on a database where if generic_asset.type = 'raw' then generic_asset.atomic = 1 must be maintained. For this I wrote the following trigger of type BEFORE INSERT. Here is the snippet:
DELIMITER //
CREATE TRIGGER generic_asset_check BEFORE INSERT ON generic_asset FOR EACH ROW
BEGIN
IF NEW.type = 'raw' THEN
BEGIN
IF NEW.atomic = 0 THEN
SET SQLSTATE = 'Sorry cannot insert';
END IF;
END IF;
END //
DELIMITER ;
Error is like:
#1064 - syntax error near 'SQLSTATE = 'Sorry cannot insert';
END IF;
END IF;
END' in line 6
(translated from French).
I tried various syntax but all seam not to work and also knowing that my changes are so little like changing double quotes, removing BEGIN with END IF;... So I know these turns are irreverent.
I revised syntax in many internet resources and official documentation, nothing helped.
MySQL's IF statement does not take a BEGIN keyword. Also, if you want to raise an error from within the trigger, you need SIGNAL. Finally, these two nested conditions can be flattened.
Consider:
DELIMITER //
CREATE TRIGGER generic_asset_check BEFORE INSERT ON generic_asset FOR EACH ROW
BEGIN
IF NEW.type = 'raw' AND NEW.atomic = 0 THEN
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Sorry cannot insert';
END IF;
END //
DELIMITER ;
Demo on DB Fiddle
delimiter //
create trigger tr_emp2 before insert or update
on employee for each row
begin
if new.ID<>10100 then
if new.Mgr not in (select ID from employee)
then
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Invalid Mgr_id!';
end if;
end if;
end;//
delimiter ;
How to express "create trigger tr_emp before insert or update" ,is it a wrong exoression ?
You have to create two triggers, but you can move the common code into a procedure and have them both call the procedure.
I am having an error and I can't make sense of it. Here is the code for my trigger:
CREATE TRIGGER before_insert_test
BEFORE INSERT ON player_totals FOR EACH ROW
BEGIN
IF NEW.Player = 'Player' THEN
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'basketball-reference duplicate header';
END IF;
END;
I am getting red errors marks in MySQL workbench source code editor for 3 lines. For the line that begins SIGNAL SQLSTATE ..., It says:
Syntax Error: missing 'semicolon'
For the line that reads END IF;, the error says:
Syntax error: END (end) is not valid input as this position.
For the line that reads END;, the error says:
Extraneous input found - expected end of statement
Just not sure how to fix these errors, from what I've seen this looks like the correct syntax...
Try this one, with $$ after END
DELIMITER $$
CREATE TRIGGER before_insert_test
BEFORE INSERT ON player_totals
FOR EACH ROW
BEGIN
IF NEW.Player = 'Player' THEN
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'basketball-reference duplicate header';
END IF;
END; $$
DELIMITER ;
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 ;
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.
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.
If I have a trigger before the update on a table, how can I throw an error that prevents the update on that table?
As of MySQL 5.5, you can use the SIGNAL syntax to throw an exception:
signal sqlstate '45000' set message_text = 'My Error Message';
State 45000 is a generic state representing "unhandled user-defined exception".
Here is a more complete example of the approach:
delimiter //
use test//
create table trigger_test
(
id int not null
)//
drop trigger if exists trg_trigger_test_ins //
create trigger trg_trigger_test_ins before insert on trigger_test
for each row
begin
declare msg varchar(128);
if new.id < 0 then
set msg = concat('MyTriggerError: Trying to insert a negative value in trigger_test: ', cast(new.id as char));
signal sqlstate '45000' set message_text = msg;
end if;
end
//
delimiter ;
-- run the following as seperate statements:
insert into trigger_test values (1), (-1), (2); -- everything fails as one row is bad
select * from trigger_test;
insert into trigger_test values (1); -- succeeds as expected
insert into trigger_test values (-1); -- fails as expected
select * from trigger_test;
Here is one hack that may work. It isn't clean, but it looks like it might work:
Essentially, you just try to update a column that doesn't exist.
Unfortunately, the answer provided by #RuiDC does not work in MySQL versions prior to 5.5 because there is no implementation of SIGNAL for stored procedures.
The solution I've found is to simulate a signal throwing a table_name doesn't exist error, pushing a customized error message into the table_name.
The hack could be implemented using triggers or using a stored procedure. I describe both options below following the example used by #RuiDC.
Using triggers
DELIMITER $$
-- before inserting new id
DROP TRIGGER IF EXISTS before_insert_id$$
CREATE TRIGGER before_insert_id
BEFORE INSERT ON test FOR EACH ROW
BEGIN
-- condition to check
IF NEW.id < 0 THEN
-- hack to solve absence of SIGNAL/prepared statements in triggers
UPDATE `Error: invalid_id_test` SET x=1;
END IF;
END$$
DELIMITER ;
Using a stored procedure
Stored procedures allows you to use dynamic sql, which makes possible the encapsulation of the error generation functionality in one procedure. The counterpoint is that we should control the applications insert/update methods, so they use only our stored procedure (not granting direct privileges to INSERT/UPDATE).
DELIMITER $$
-- my_signal procedure
CREATE PROCEDURE `my_signal`(in_errortext VARCHAR(255))
BEGIN
SET #sql=CONCAT('UPDATE `', in_errortext, '` SET x=1');
PREPARE my_signal_stmt FROM #sql;
EXECUTE my_signal_stmt;
DEALLOCATE PREPARE my_signal_stmt;
END$$
CREATE PROCEDURE insert_test(p_id INT)
BEGIN
IF NEW.id < 0 THEN
CALL my_signal('Error: invalid_id_test; Id must be a positive integer');
ELSE
INSERT INTO test (id) VALUES (p_id);
END IF;
END$$
DELIMITER ;
The following procedure is (on mysql5) a way to throw custom errors , and log them at the same time:
create table mysql_error_generator(error_field varchar(64) unique) engine INNODB;
DELIMITER $$
CREATE PROCEDURE throwCustomError(IN errorText VARCHAR(44))
BEGIN
DECLARE errorWithDate varchar(64);
select concat("[",DATE_FORMAT(now(),"%Y%m%d %T"),"] ", errorText) into errorWithDate;
INSERT IGNORE INTO mysql_error_generator(error_field) VALUES (errorWithDate);
INSERT INTO mysql_error_generator(error_field) VALUES (errorWithDate);
END;
$$
DELIMITER ;
call throwCustomError("Custom error message with log support.");
CREATE TRIGGER sample_trigger_msg
BEFORE INSERT
FOR EACH ROW
BEGIN
IF(NEW.important_value) < (1*2) THEN
DECLARE dummy INT;
SELECT
Enter your Message Here!!!
INTO dummy
FROM mytable
WHERE mytable.id=new.id
END IF;
END;
Another (hack) method (if you are not on 5.5+ for some reason) that you can use:
If you have a required field, then within a trigger set the required field to an invalid value such as NULL. This will work for both INSERT and UPDATE. Do note that if NULL is a valid value for the required field (for some crazy reason) then this approach will not work.
BEGIN
-- Force one of the following to be assigned otherwise set required field to null which will throw an error
IF (NEW.`nullable_field_1` IS NULL AND NEW.`nullable_field_2` IS NULL) THEN
SET NEW.`required_id_field`=NULL;
END IF;
END
If you are on 5.5+ then you can use the signal state as described in other answers:
BEGIN
-- Force one of the following to be assigned otherwise use signal sqlstate to throw a unique error
IF (NEW.`nullable_field_1` IS NULL AND NEW.`nullable_field_2` IS NULL) THEN
SIGNAL SQLSTATE '45000' set message_text='A unique identifier for nullable_field_1 OR nullable_field_2 is required!';
END IF;
END
DELIMITER ##
DROP TRIGGER IF EXISTS trigger_name ##
CREATE TRIGGER trigger_name
BEFORE UPDATE ON table_name
FOR EACH ROW
BEGIN
--the condition of error is:
--if NEW update value of the attribute age = 1 and OLD value was 0
--key word OLD and NEW let you distinguish between the old and new value of an attribute
IF (NEW.state = 1 AND OLD.state = 0) THEN
signal sqlstate '-20000' set message_text = 'hey it's an error!';
END IF;
END ##
DELIMITER ;