HiveBrain v1.2.0
Get Started
← Back to all entries
debugsqlMinor

Foreign Key Constraint fails

Submitted by: @import:stackexchange-dba··
0
Viewed 0 times
failsforeignkeyconstraint

Problem

I have the following tables:

// Base Scans
CREATE TABLE `basescans` (
    `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `name` VARCHAR(100) NULL DEFAULT NULL,
    `status_id` INT(10) UNSIGNED NULL DEFAULT NULL,
    PRIMARY KEY (`id`),
    CONSTRAINT `status_id_fk` FOREIGN KEY (`status_id`) REFERENCES `statuses` (`id`) ON UPDATE CASCADE ON DELETE SET NULL
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
ROW_FORMAT=COMPACT
AUTO_INCREMENT=29

// Statuses
CREATE TABLE `statuses` (
    `id` INT(10) UNSIGNED NULL AUTO_INCREMENT,
    `name` VARCHAR(100) NULL DEFAULT NULL,
    PRIMARY KEY (`id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
ROW_FORMAT=DEFAULT
AUTO_INCREMENT=4


Trying to save the first table fails when I put in that foreign key constraint. Can't figure out why. Both of the columns referenced in the constraint have the same type, size, etc:

INT(10) UNSIGNED NULL


They only have a difference default value. One has a default value of NULL, the other is AUTO_INCREMENT. I didn't think that made a difference for foreign key constraints but I could be wrong.

Both tables are InnoDB and UFT8. What am I missing here?

UPDATED: My specific error:

/* SQL Error (1452): Cannot add or update a child row: a foreign key constraint fails (`db`., CONSTRAINT `status_id_fk` FOREIGN KEY (`status_id`) REFERENCES `statuses` (`id`) ON DELETE SET NULL ON UPDATE CASCADE) */

Solution

You need to have already declared the table that the foreign key references, before you can define a foreign key that references it.

Once you declare the second table, you can then declare the first table. Tested here on MySQL 5.5.27.

If you need to bypass the validation, you can do this:

SET FOREIGN_KEY_CHECKS = 0;
-- declare tables
SET FOREIGN_KEY_CHECKS = 1;


http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_foreign_key_checks

Code Snippets

SET FOREIGN_KEY_CHECKS = 0;
-- declare tables
SET FOREIGN_KEY_CHECKS = 1;

Context

StackExchange Database Administrators Q#25581, answer score: 6

Revisions (0)

No revisions yet.