MSSQL Try Catch and Set Identity Insert in MySQL? - mysql

Question

Do you know how to rewrite these this query in MySQL ?
I can't find Identity insert, I can't find any try catch,
I don't understand it.
CREATE TRIGGER T1 ON DB1.dbo.A
AFTER INSERT AS
BEGIN TRY
SET IDENTITY_INSERT DB2.dbo.B ON
INSERT INTO dbo.B(id, text) SELECT A.id,A.text FROM dbo.A INNER JOIN inserted I ON I.id = A.id
SET IDENTITY_INSERT DB2.dbo.B OFF
SET IDENTITY_INSERT DB2.dbo.D ON
INSERT INTO dbo.D(id, text) SELECT A.id,A.text FROM dbo.A INNER JOIN inserted I ON I.id = A.id
SET IDENTITY_INSERT DB2.dbo.D OFF
END TRY
BEGIN CATCH
ROLLBACK TRANSACTION
SET IDENTITY_INSERT DB2.dbo.B OFF
SET IDENTITY_INSERT DB2.dbo.D OFF
END CATCH
GO

Answer 1

MySQL triggers have implicit transaction support, so the trigger cannot use statements that explicitly or implicitly begin or end a transaction such as START TRANSACTION, COMMIT, or ROLLBACK.
It is not necessary in MySQL to enable the insertion of values into primary key columns - this is already allowed. You can, however, toggle foreign key constraint checking and unique index checking:
http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html
http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_unique_checks
A common way to do this is to store the existing values in user variables, change the settings, then restore the settings after your script is complete:
SET #OLD_UNIQUE_CHECKS=##UNIQUE_CHECKS, UNIQUE_CHECKS=0;
SET #OLD_FOREIGN_KEY_CHECKS=##FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0;
-- Your SQL statements here.
SET FOREIGN_KEY_CHECKS=#OLD_FOREIGN_KEY_CHECKS;
SET UNIQUE_CHECKS=#OLD_UNIQUE_CHECKS;
I'm not sure why you would need to do that in your trigger, so your MySQL trigger would look something like this:
DELIMITER |
CREATE TRIGGER T1 AFTER INSERT ON A FOR EACH ROW
BEGIN
INSERT INTO B (id, text) VALUES (NEW.id, NEW.text);
INSERT INTO C (id, text) VALUES (NEW.id, NEW.text);
END;|
DELIMITER ;
Here's the results of a quick test:
CREATE TABLE `A` (
`id` int(11) NOT NULL auto_increment,
`text` varchar(255) default NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `B` (
`id` int(11) NOT NULL auto_increment,
`text` varchar(255) default NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `C` (
`id` int(11) NOT NULL auto_increment,
`text` varchar(255) default NULL,
PRIMARY KEY (`id`)
);
DELIMITER |
CREATE TRIGGER T1 AFTER INSERT ON A FOR EACH ROW
BEGIN
INSERT INTO B (id, text) VALUES (NEW.id, NEW.text);
INSERT INTO C (id, text) VALUES (NEW.id, NEW.text);
END;|
DELIMITER ;
INSERT INTO `A` (id, text) VALUES (1, 'Line 1');
INSERT INTO `A` (id, text) VALUES (2, 'Line 3');
INSERT INTO `A` (id, text) VALUES (3, 'Line 3');
SELECT * FROM `A`;
+----+--------+
| id | text |
+----+--------+
| 1 | Line 1 |
| 2 | Line 3 |
| 3 | Line 3 |
+----+--------+
SELECT * FROM `B`;
+----+--------+
| id | text |
+----+--------+
| 1 | Line 1 |
| 2 | Line 3 |
| 3 | Line 3 |
+----+--------+
SELECT * FROM `C`;
+----+--------+
| id | text |
+----+--------+
| 1 | Line 1 |
| 2 | Line 3 |
| 3 | Line 3 |
+----+--------+
If you want something similar to TRY ... CATCH, you'll need to use handlers instead:
http://dev.mysql.com/doc/refman/5.1/en/declare-handler.html
Here's the documentation on MySQL triggers:
http://dev.mysql.com/doc/refman/5.1/en/commit.html

Answer 2

Just set id column to AUTO_INCREMENT. You don't need to toggle something on and off.
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