圓形外鍵約束失敗mysql

Question

如果我在InnoDB引擎上設計數據庫，並且我有3個表，我無法刪除，因爲每個表示「外鍵約束失敗」 - 是否意味着設計錯誤？

請參閱下面的結構：

CREATE TABLE IF NOT EXISTS `account` (
    `account_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT, 
    PRIMARY KEY (`account_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ; 


CREATE TABLE IF NOT EXISTS `identity` (
    `identity_id` int(10) unsigned NOT NULL AUTO_INCREMENT, 
    `user_id` int(10) unsigned DEFAULT NULL, 
    `account_id` smallint(5) unsigned DEFAULT NULL, 
    PRIMARY KEY (`identity_id`), 
    KEY `fk_details1` (`user_id`), 
    KEY `fk_account1` (`account_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ; 


CREATE TABLE IF NOT EXISTS `user` (
    `user_id` int(10) unsigned NOT NULL AUTO_INCREMENT, 
    `login` varchar(64) NOT NULL DEFAULT '', 
    `password` varchar(32) NOT NULL DEFAULT '', 
    `default_identity_id` int(10) unsigned DEFAULT NULL, 
    PRIMARY KEY (`user_id`), 
    UNIQUE KEY `login_UNIQUE` (`login`), 
    KEY `fk_identity1` (`default_identity_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ; 

-- 
-- Constraints for table `identity` 
-- 
ALTER TABLE `identity` 
    ADD CONSTRAINT `fk_details1` FOREIGN KEY (`user_id`) REFERENCES `user` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE, 
    ADD CONSTRAINT `fk_account1` FOREIGN KEY (`account_id`) REFERENCES `account` (`account_id`) ON DELETE CASCADE ON UPDATE CASCADE; 

-- 
-- Constraints for table `user` 
-- 
ALTER TABLE `user` 
    ADD CONSTRAINT `fk_identity1` FOREIGN KEY (`default_identity_id`) REFERENCES `identity` (`identity_id`) ON DELETE CASCADE ON UPDATE CASCADE; 

我懷疑問題是default_identity_id ...我將它作爲一個標誌，標識表？

請指教！

Answer 1

由於ypercube沒有創建答案，我會的。

MySQL Database design. Inserting rows in 1to1 tables

這回答我的問題 - 它不僅僅是指向表彼此更好的和靈活的設計。謝謝。