MySQL - Why can't I add a foreign key here?

Question

I'm not sure why I'm not able to add a particular foreign key here. These tables were generated via MySQL Workbench, and based on the MySQL documentation and on searching for other similar problems / solutions, I think everything is in order...yet I can't add one foreign key in particular:

inspection_statuses . inspection_id needs to reference inspection_responses . tpa_result

What am I missing / overlooking?

This is the statement I'm trying to use to add the new foreign key:

ALTER TABLE `vipsouth_app`.`inspection_statuses` 
ADD CONSTRAINT `inspection_statuses_ibfk_3`
  FOREIGN KEY (`inspection_id`)
  REFERENCES `vipsouth_app`.`inspection_responses` (`tpa_result`)
  ON DELETE NO ACTION
  ON UPDATE NO ACTION;

And that produces this error:

Operation failed: There was an error while applying the SQL script to the database. ERROR 1005: Can't create table vipsouth_app . #sql-1f48_7 (errno: 150 "Foreign key constraint is incorrectly formed")

CREATE TABLE `inspection_responses` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `inspection_request_id` int(10) unsigned NOT NULL,
  `tpa_result` varchar(10) CHARACTER SET utf8 NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `created_by` int(10) unsigned NOT NULL DEFAULT '0',
  `updated_at` timestamp NULL DEFAULT NULL,
  `updated_by` int(10) unsigned DEFAULT NULL,
  `deleted_at` timestamp NULL DEFAULT NULL,
  `deleted_by` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id_UNIQUE` (`id`),
  UNIQUE KEY `tpa_result_UNIQUE` (`tpa_result`),
  KEY `inspection_responses_ibfk_1_idx` (`inspection_request_id`),
  CONSTRAINT `inspection_responses_ibfk_1` FOREIGN KEY (`inspection_request_id`) REFERENCES `inspection_requests` (`id`) ON DELETE     CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

CREATE TABLE `inspection_statuses` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `inspection_request_id` int(10) unsigned NOT NULL,
  `inspection_response_id` int(10) unsigned NOT NULL,
  `tpa_code` varchar(4) COLLATE utf8_unicode_ci NOT NULL,
  `user_id` varchar(30) COLLATE utf8_unicode_ci NOT NULL,
  `password` varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  `inspection_id` varchar(10) COLLATE utf8_unicode_ci NOT NULL,
  `status` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
  `note` varchar(1000) COLLATE utf8_unicode_ci DEFAULT NULL,
  `url` varchar(1024) COLLATE utf8_unicode_ci DEFAULT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `created_by` int(10) unsigned NOT NULL,
  `updated_at` timestamp NULL DEFAULT NULL,
  `updated_by` int(10) unsigned DEFAULT NULL,
  `deleted_at` timestamp NULL DEFAULT NULL,
  `deleted_by` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id_UNIQUE` (`id`),
  KEY `inspection_statuses_ibfk_1_idx` (`inspection_request_id`),
  KEY `inspection_statuses_ibfk_2_idx` (`inspection_response_id`),
  CONSTRAINT `inspection_statuses_ibfk_1` FOREIGN KEY (`inspection_request_id`) REFERENCES `inspection_requests` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `inspection_statuses_ibfk_2` FOREIGN KEY (`inspection_response_id`) REFERENCES `inspection_responses` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

Answer 1

The data types and constraints defined on both the columns should be exactly the same. Except for a foreign key can be NULLable.

This should do it for you. I did not get any error.

SET foreign_key_checks = 0;

    CREATE TABLE `inspection_responses` (
      `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
      `inspection_request_id` int(10) unsigned NOT NULL,
      `tpa_result` varchar(10) CHARACTER SET utf8 NOT NULL,
      `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
      `created_by` int(10) unsigned NOT NULL DEFAULT '0',
      `updated_at` timestamp NULL DEFAULT NULL,
      `updated_by` int(10) unsigned DEFAULT NULL,
      `deleted_at` timestamp NULL DEFAULT NULL,
      `deleted_by` int(10) unsigned DEFAULT NULL,
      PRIMARY KEY (`id`),
      UNIQUE KEY `id_UNIQUE` (`id`),
      UNIQUE KEY `tpa_result_UNIQUE` (`tpa_result`),
      KEY `inspection_responses_ibfk_1_idx` (`inspection_request_id`),
      CONSTRAINT `inspection_responses_ibfk_1` FOREIGN KEY (`inspection_request_id`) REFERENCES `inspection_requests` (`id`) ON DELETE     CASCADE ON UPDATE CASCADE
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

    CREATE TABLE `inspection_statuses` (
      `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
      `inspection_request_id` int(10) unsigned NOT NULL,
      `inspection_response_id` int(10) unsigned NOT NULL,
      `tpa_code` varchar(4) COLLATE utf8_unicode_ci NOT NULL,
      `user_id` varchar(30) COLLATE utf8_unicode_ci NOT NULL,
      `password` varchar(100) COLLATE utf8_unicode_ci NOT NULL,
      `inspection_id` varchar(10) COLLATE utf8_unicode_ci NOT NULL,
      `status` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
      `note` varchar(1000) COLLATE utf8_unicode_ci DEFAULT NULL,
      `url` varchar(1024) COLLATE utf8_unicode_ci DEFAULT NULL,
      `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
      `created_by` int(10) unsigned NOT NULL,
      `updated_at` timestamp NULL DEFAULT NULL,
      `updated_by` int(10) unsigned DEFAULT NULL,
      `deleted_at` timestamp NULL DEFAULT NULL,
      `deleted_by` int(10) unsigned DEFAULT NULL,
      PRIMARY KEY (`id`),
      UNIQUE KEY `id_UNIQUE` (`id`),
      KEY `inspection_statuses_ibfk_1_idx` (`inspection_request_id`),
      KEY `inspection_statuses_ibfk_2_idx` (`inspection_response_id`),
      CONSTRAINT `inspection_statuses_ibfk_1` FOREIGN KEY (`inspection_request_id`) REFERENCES `inspection_requests` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
      CONSTRAINT `inspection_statuses_ibfk_2` FOREIGN KEY (`inspection_response_id`) REFERENCES `inspection_responses` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

SET foreign_key_checks = 1;