MySQL Trigger To Update New Row
We are in the process of migrating between 2 systems and need to have 2 fields for one of our database tables that always stay in sync. Here is the table structure:
CREATE TABLE `example` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`object_id` int(11) NOT NULL DEFAULT '0',
`value` varchar(200) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `object_id` (`object_id`)
);
Every time one of the systems inserts a new row we need to have object_id set to id. We can't use 'before insert' since the id column is an auto_increment column so it's value is NULL before insert and due to the limitations of the MySQL 'after insert' on triggers I can't do the following:
CREATE TRIGGER insert_example
AFTER INSERT ON example
FOR EACH ROW
SET 开发者_开发问答NEW.object_id = NEW.id;
I can't update the code for either system so I need a way to accomplish this on the database side. Both systems are going to be inserting new rows. How can I accomplish this?
Using a trigger which fires before the insert should do the job
CREATE TRIGGER insert_example
BEFORE INSERT ON example
FOR EACH ROW
SET NEW.object_id = NEW.id;
EDIT:
As the OP pointed out NEW.id won't work with auto-increment; one could use the following trigger (use at own risk):
CREATE TRIGGER insert_example
BEFORE INSERT ON example
FOR EACH ROW
SET NEW.object_id = (
SELECT AUTO_INCREMENT
FROM information_schema.TABLES
WHERE TABLE_SCHEMA = DATABASE()
AND TABLE_NAME = 'example'
);
But I'd rather re-think this somewhat strange requirement - why do you need the pk value twice in the table?
Is there any reason you cant use a BEFORE INSERT
trigger?
I've always seen AFTER INSERT
triggers as a method to manipulate other tables rather than the table for which the trigger was executed on.
Rule of thumb, manipulate table the trigger is running on = BEFORE INSERT
, manipulate other tables AFTER INSERT
:)
I think your trigger will never create in the first place because you can't refer NEW.column_name in an AFTER INSERT trigger.
Try doing this in a BEFORE INSERT trigger (PLEASE IGNORE THIS FIX AS IT WILL NOT WORK):
CREATE TRIGGER `insert_example` BEFORE INSERT ON `t`
FOR EACH ROW
SET NEW.`object_id` = NEW.`id`;
Please change the table and column names as per your schema.
Hope this helps.
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