CREATE TABLE `parent_blocks`
(
    `block_id`        BIGINT UNSIGNED NOT NULL,
    `parent_block_id` BIGINT UNSIGNED NOT NULL,
    PRIMARY KEY (`block_id`, `parent_block_id`),
    CONSTRAINT `fk_parent_blocks_block_id`
        FOREIGN KEY (`block_id`)
            REFERENCES `blocks` (`id`),
    CONSTRAINT `fk_parent_blocks_parent_block_id`
        FOREIGN KEY (`parent_block_id`)
            REFERENCES `blocks` (`id`)
);