Question

如何建模ISA类？一个'Persoon'ISA'学生'或'Docent'，你如何在MySQL中建模？我尝试使用2个字段制作Persoon课程，然后您可以将学生/讲师的ID放在正确的字段中。另一个是零。

CREATE TABLE IF NOT EXISTS `Docent` (
  `docent_id` int(4) unsigned NOT NULL AUTO_INCREMENT,,
  PRIMARY KEY (`docent_id`),
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

-- --------------------------------------------------------

CREATE TABLE IF NOT EXISTS `Persoon` (
  `naam` varchar(50) NOT NULL,
  `email` varchar(50) NOT NULL,
  `wachtwoord` varchar(100) NOT NULL,
  `student_id` int(4) unsigned DEFAULT NULL,
  `docent_id` int(4) unsigned DEFAULT NULL,
  PRIMARY KEY (`email`),
  KEY `email` (`email`),
  KEY `student_id` (`student_id`),
  KEY `docent_id` (`docent_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- --------------------------------------------------------

CREATE TABLE IF NOT EXISTS `Student` (
  `student_id` int(4) unsigned NOT NULL AUTO_INCREMENT,
  `studentnummer` int(20) NOT NULL,
  `niveau` int(2) NOT NULL,
  `notities` varchar(200) NOT NULL,
  PRIMARY KEY (`student_id`),
  UNIQUE KEY `email` (`email`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

添加外键时，如下所示，我收到＃1452错误。我查了一下，但没找到如何解决它。

ALTER TABLE  `Persoon` ADD CONSTRAINT  `Persoons_student_id` FOREIGN KEY (  `student_id` ) REFERENCES  `software`.`Student` (`student_id`) ON DELETE CASCADE ON UPDATE CASCADE ;
ALTER TABLE  `Persoon` ADD CONSTRAINT  `Persoons_docent_id` FOREIGN KEY (  `docent_id` ) REFERENCES  `software`.`Docent` (`docent_id`) ON DELETE CASCADE ON UPDATE CASCADE ;

给出了这个错误：

#1452 - Cannot add or update a child row: a foreign key constraint fails (`software`.<result 2 when explaining filename '#sql-c0_54eea'>, CONSTRAINT `Persoons_student_id` FOREIGN KEY (`student_id`) REFERENCES `Student` (`student_id`) ON DELETE CASCADE ON UPDATE CASC)

Answer 1

你正在倒退 - 来自＆＃34;特定＆＃34;的外键。表格应该参考＆＃34; general＆＃34;表

在您的情况下：Docent应引用Person，Student也应引用Person，类似于：

CREATE TABLE IF NOT EXISTS Person (
  person_id int(4) unsigned AUTO_INCREMENT,
  -- Other fields...
  PRIMARY KEY (person_id)
);

CREATE TABLE IF NOT EXISTS Student (
  student_id int(4) unsigned,
  -- Other fields...
  PRIMARY KEY (student_id),
  FOREIGN KEY (student_id)
    REFERENCES Person (person_id)
    ON DELETE CASCADE ON UPDATE CASCADE
);

CREATE TABLE IF NOT EXISTS Docent (
  docent_id int(4) unsigned,
  -- Other fields...
  PRIMARY KEY (docent_id),
  FOREIGN KEY (docent_id)
    REFERENCES Person (person_id)
    ON DELETE CASCADE ON UPDATE CASCADE
);

通过以下方式插入新学生

首先在Person中添加一行并获取生成的person_id（例如，通过LAST_INSERT_ID）。
而不是在student_id中插入行时为Student使用相同的值。

Ditto for docent。

使用外键在MySQL中建立ISA关系模型

1 个答案: