mysql cannot add foreign key for unknown reason - mysql

Question

I cannot a add a foreign key constraint. the sql i m running is -
ALTER TABLE image_shout ADD CONSTRAINT `fk_image` FOREIGN KEY (image_id)
REFERENCES images(image_id);
the collation and the data types( int(10) ) are same in the two tables.
mysql says -
Error Code: 1215. Cannot add foreign key constraint
The images table structure -
CREATE TABLE `images` (
`image_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`image_name` varchar(100) CHARACTER SET latin1 NOT NULL,
`created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`caption` varchar(450) CHARACTER SET latin1 DEFAULT NULL,
`image_visibility` int(10) unsigned NOT NULL,
`album_id` int(10) unsigned NOT NULL,
`album_view` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`album_thumb_view` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
PRIMARY KEY (`image_id`),
KEY `Index_2` (`album_id`),
CONSTRAINT `FK_images_1` FOREIGN KEY (`album_id`) REFERENCES `photo_album` (`Album_ID`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4314 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
The image_shout table -
CREATE TABLE `image_shout` (
`auto_id` int(11) NOT NULL AUTO_INCREMENT,
`shout_id` int(11) DEFAULT NULL,
`image_id` int(10) DEFAULT NULL,
PRIMARY KEY (`auto_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1132 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
UPDATE -
The new error after changing the image_id column to unsigned is -
Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails
(`void`.`#sql-36b_7285`, CONSTRAINT `fk_image` FOREIGN KEY (`image_id`)
REFERENCES `images` (`image_id`) ON DELETE SET NULL ON UPDATE CASCADE)
Regards

Answer 1

Is because the image_id in table images is defined as unsigned
`image_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
and in image_shout not
`image_id` int(10) DEFAULT NULL,
Change both columns to the same data type and it should work.

Answer 2

The problem is below. According to the documentation, a foreign key must be indexed. Further, a foreign key should reference the KEY of another table. Not just a column. Try using auto_id as the constraint if you don't wish to change your table structure.
Please see MySQL Documentation for the list of requirements of a Foreign Key constraint.
CREATE TABLE `image_shout` (
`auto_id` int(11) NOT NULL AUTO_INCREMENT,
`shout_id` int(11) DEFAULT NULL,
`image_id` int(10) DEFAULT NULL,
PRIMARY KEY (`auto_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1132 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;