I have the following table definition:

CREATE TABLE `async_task` (
  `idasync_task` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `idasync_type` int(10) unsigned NOT NULL,
  `priority` tinyint(3) NOT NULL,
  `status` enum('todo','doing','failed') NOT NULL DEFAULT 'todo',
  `iduser` int(11) NOT NULL,
  `date_added` datetime NOT NULL,
  PRIMARY KEY (`idasync_task`),
  KEY `priority_id` (`priority`,`idasync_task`),
  KEY `status_type` (`status`,`idasync_type`)

I've added a column to the my notification table which I want to point to async_task:

ALTER TABLE `notification` ADD COLUMN `async_task_id` BIGINT(20)

And when I add the following foreign key:

ALTER TABLE `notification` ADD CONSTRAINT `fk_notification_async_task` 
    FOREIGN KEY (`async_task_id`) REFERENCES `async_task`(`idasync_task`);

I get:

ERROR 1005 (HY000): Can't create table `my_database`.`#sql-182_2d` 
(errno: 150 "Foreign key constraint is incorrectly formed")

I've looked elsewhere but only find the errors as being:

  1. The table you're referencing is not created (not the case)
  2. The table you're referencing is not InnoDB (not the case, both notification and async_task are InnoDB)
  3. You're not referencing the entire primary key (not the case, the only primary key is the ID column).

What else could it be?

  • 2
    Please add notification table schema. – McNets Apr 10 '18 at 14:19

Both the referencing and referenced columns must be of the same type (and same in this case includes the unsigned attribute).

You didn't define notification.async_task_id as unsigned, so it was created with the (default) signed. Fix that and the foreign key will raise no errors.

  • I found that i could also be linked to a difference between the default charset ,collation and the one defined in the CREATE statement. – Raymond Aug 30 '19 at 13:15

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