SQLError - create foreign key - mysql

Question

An error occurred when trying to add a foreign key
SQLError
Before that, the following actions were performed, indexes were installed
maintable
subordinatetable
Acted according to https://folkprog.net/sozdanie-svyazey-phpmyadmin/.
Error text:
ALTER TABLE monoblocks ADD FOREIGN KEY (image_id) REFERENCES monoblock_images(id) ON DELETE RESTRICT ON UPDATE RESTRICT;
#1452 - Cannot add or update a child row: a foreign key constraint fails (myishop.#sql-2150_93, CONSTRAINT #sql-2150_93_ibfk_21 FOREIGN KEY (image_id) REFERENCES monoblock_images (id))
I have found
CREATE TABLE `monoblocks` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`goods_id` int(11) NOT NULL,
`screendiagonal_id` int(11) NOT NULL,
`cpu_id` int(11) NOT NULL,
`ramsize_id` int(11) NOT NULL,
`os_id` int(11) NOT NULL,
`hdd_id` int(11) NOT NULL,
`screenresolution_id` int(11) NOT NULL,
`matrixtype_id` int(11) NOT NULL,
`countcpucores` int(11) NOT NULL,
`videocards_id` int(11) NOT NULL,
`videocardtypes_id` int(11) NOT NULL,
`frequency_id` int(11) NOT NULL,
`ramtype_id` int(11) NOT NULL,
`oddtype_id` int(11) NOT NULL,
`bodycolor_id` int(11) NOT NULL,
`weight` int(11) NOT NULL,
`wifisupport_id` int(11) NOT NULL,
`bluetoothsupport_id` int(11) NOT NULL,
`wifistandard_id` int(11) NOT NULL,
`bluetoothstandard_id` int(11) NOT NULL,
`guarantee_id` int(11) NOT NULL,
`connectorsnumberusb2.0` int(11) NOT NULL,
`connectorsnumberusb3.0` int(11) NOT NULL,
`connectorsnumberhdmi` int(11) NOT NULL,
`microphoneconnecors` int(11) NOT NULL,
`headphoneconnectors` int(11) NOT NULL,
`builtinmicrophone` tinyint(1) NOT NULL,
`builtinspeakers` tinyint(1) NOT NULL,
`touchscreen` tinyint(1) NOT NULL,
`webcam` tinyint(1) NOT NULL,
`keyboard_included` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
`mouse_included` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
`ssd_volume` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
`dimensions` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
`image_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `id` (`id`),
KEY `goods_id` (`goods_id`),
KEY `screendiagonal_id` (`screendiagonal_id`),
KEY `cpu_id` (`cpu_id`),
KEY `ramsize_id` (`ramsize_id`),
KEY `os_id` (`os_id`),
KEY `hdd` (`hdd_id`),
KEY `screenresolution_id` (`screenresolution_id`),
KEY `matrixtype_id` (`matrixtype_id`),
KEY `countcpucores` (`countcpucores`),
KEY `videocards_id` (`videocards_id`),
KEY `videocardtypes_id` (`videocardtypes_id`),
KEY `frequency_id` (`frequency_id`),
KEY `ramtype_id` (`ramtype_id`),
KEY `oddtype_id` (`oddtype_id`),
KEY `bodycolor_id` (`bodycolor_id`),
KEY `wifisupport_id` (`wifisupport_id`),
KEY `bluetoothsupport_id` (`bluetoothsupport_id`),
KEY `wifistandard_id` (`wifistandard_id`),
KEY `bluetoothstandard_id` (`bluetoothstandard_id`),
KEY `guarantee_id` (`guarantee_id`),
KEY `image_id` (`image_id`),
CONSTRAINT `monoblocks_ibfk_1` FOREIGN KEY (`goods_id`) REFERENCES `goods` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_10` FOREIGN KEY (`videocards_id`) REFERENCES `videocards` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_11` FOREIGN KEY (`videocardtypes_id`) REFERENCES `videocardtypes` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_12` FOREIGN KEY (`frequency_id`) REFERENCES `cpufrequency` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_13` FOREIGN KEY (`ramtype_id`) REFERENCES `rammemory` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_14` FOREIGN KEY (`oddtype_id`) REFERENCES `oddtype` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_15` FOREIGN KEY (`bodycolor_id`) REFERENCES `mainbodycolor` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_16` FOREIGN KEY (`wifisupport_id`) REFERENCES `wifisupport` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_17` FOREIGN KEY (`bluetoothsupport_id`) REFERENCES `bluetoothsupport` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_18` FOREIGN KEY (`wifistandard_id`) REFERENCES `wifistandard` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_19` FOREIGN KEY (`bluetoothstandard_id`) REFERENCES `bluetoothstandard` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_2` FOREIGN KEY (`screendiagonal_id`) REFERENCES `screendiagonal` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_20` FOREIGN KEY (`guarantee_id`) REFERENCES `guarantee` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_3` FOREIGN KEY (`cpu_id`) REFERENCES `cpus` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_4` FOREIGN KEY (`ramsize_id`) REFERENCES `ramsize` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_5` FOREIGN KEY (`os_id`) REFERENCES `os` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_6` FOREIGN KEY (`hdd_id`) REFERENCES `disksandrives` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_7` FOREIGN KEY (`screenresolution_id`) REFERENCES `screenresolution` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_8` FOREIGN KEY (`matrixtype_id`) REFERENCES `matrixtype` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `monoblocks_ibfk_9` FOREIGN KEY (`countcpucores`) REFERENCES `numbercpucores` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
CREATE TABLE `monoblock_images` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`path_to_image` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
(may be help to answer the question)
In answer on SELECT mb.image_id FROM monoblocks AS mb WHERE NOT EXISTS (SELECT 1 FROM monoblock_images AS i WHERE i.id = mb.image_id) AND mb.image_id IS NOT NULL I gave
monoblockquery

Answer 1

image_id can be NULL in the Table monoblocks but not in the other Table.