Mysql 错误1822（HY000）：未能添加外键约束。SQL中缺少约束问题的索引

我创建了一个数据库，创建了3个表（

类、讲座、取

），然后修改表以添加

外键

但是，我不断遇到以下错误：

错误1822（HY000）：未能添加外键约束。缺少约束的索引

我似乎没有发现代码有任何问题，那么问题出在哪里？？ （我使用的是mySQL Workbench。我尝试将其复制和粘贴为txt文件，因为我认为这可能是排序问题，但仍然会出现相同的问题）

代码如下：

CREATE TABLE classes (
    course_id VARCHAR(8),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    PRIMARY KEY (course_id, classes_id, semester, year)
);

CREATE TABLE taking (
    student_id VARCHAR(8),
    course_id VARCHAR(10),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    grade char(1),
    PRIMARY KEY (student_id, course_id, semester, year)
);

CREATE TABLE lectures (
    professor_id VARCHAR(8),
    course_id VARCHAR(10),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    PRIMARY KEY (professor_id, course_id, semester, year)
);

ALTER TABLE taking ADD CONSTRAINT consTAKE3 FOREIGN KEY(classes_id) REFERENCES classes(classes_id) ON DELETE CASCADE;
ALTER TABLE taking ADD CONSTRAINT consTAKE4 FOREIGN KEY(semester) REFERENCES classes(semester) ON DELETE CASCADE;
ALTER TABLE taking ADD CONSTRAINT consTAKE5 FOREIGN KEY(year) REFERENCES classes(year) ON DELETE CASCADE;

ALTER TABLE lectures ADD CONSTRAINT consLEC3 FOREIGN KEY(classes_id) REFERENCES classes(classes_id) ON DELETE CASCADE;
ALTER TABLE lectures ADD CONSTRAINT consLEC4 FOREIGN KEY(semester) REFERENCES classes(semester) ON DELETE CASCADE;
ALTER TABLE lectures ADD CONSTRAINT consLEC5 FOREIGN KEY(year) REFERENCES classes(year) ON DELETE CASCADE;

ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE3' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE4' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE5' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC3' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC4' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC5' in the referenced table 'classes'

错误如下：

CREATE TABLE classes (
    course_id VARCHAR(8),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    PRIMARY KEY (course_id, classes_id, semester, year)
);

CREATE TABLE taking (
    student_id VARCHAR(8),
    course_id VARCHAR(10),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    grade char(1),
    PRIMARY KEY (student_id, course_id, semester, year)
);

CREATE TABLE lectures (
    professor_id VARCHAR(8),
    course_id VARCHAR(10),
    classes_id VARCHAR(10),
    semester VARCHAR(10),
    year VARCHAR(10),
    PRIMARY KEY (professor_id, course_id, semester, year)
);

ALTER TABLE taking ADD CONSTRAINT consTAKE3 FOREIGN KEY(classes_id) REFERENCES classes(classes_id) ON DELETE CASCADE;
ALTER TABLE taking ADD CONSTRAINT consTAKE4 FOREIGN KEY(semester) REFERENCES classes(semester) ON DELETE CASCADE;
ALTER TABLE taking ADD CONSTRAINT consTAKE5 FOREIGN KEY(year) REFERENCES classes(year) ON DELETE CASCADE;

ALTER TABLE lectures ADD CONSTRAINT consLEC3 FOREIGN KEY(classes_id) REFERENCES classes(classes_id) ON DELETE CASCADE;
ALTER TABLE lectures ADD CONSTRAINT consLEC4 FOREIGN KEY(semester) REFERENCES classes(semester) ON DELETE CASCADE;
ALTER TABLE lectures ADD CONSTRAINT consLEC5 FOREIGN KEY(year) REFERENCES classes(year) ON DELETE CASCADE;

ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE3' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE4' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consTAKE5' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC3' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC4' in the referenced table 'classes'
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'consLEC5' in the referenced table 'classes'

---

现在，我正在尝试使用

ysql–u root–p

从

cmd

执行此操作。但是，同样的问题仍然存在。

因为所有表中都有一个复合主键，所以所有引用还必须包括表中的所有复合键列才能正确映射

除非对每个表的复合主键列具有唯一的约束/索引

要在表中创建关系，请对

dbo尝试此操作。对于其他表，也可以采用类似的方法

ALTER TABLE dbo.taking 
  ADD CONSTRAINT FK_taking_classes
  FOREIGN KEY(course_id, classes_id, semester, year) REFERENCES classes(course_id, 
classes_id, semester, year)

  ALTER TABLE dbo.taking 
  ADD CONSTRAINT FK_taking_classes
  FOREIGN KEY(course_id, classes_id, semester, year) REFERENCES classes(course_id, 
  classes_id, semester, year)

两个表中
课程id
的字符长度不同，导致不创建FK。请将其设置为
varchar（8）
中的
taking
表中的
classes
。这肯定会解决问题。我已经使用提供的查询创建了表和FKs

然后按照上述查询进行操作：

由于所有表中都有一个复合主键，所以
引用还必须包括
要正确映射的表

除非复合主键上有唯一的约束/索引
每个表上的列

要在表中创建关系，请对
dbo尝试此操作
其他表也采用类似的方法

ALTER TABLE dbo.taking 
  ADD CONSTRAINT FK_taking_classes
  FOREIGN KEY(course_id, classes_id, semester, year) REFERENCES classes(course_id, 
classes_id, semester, year)

  ALTER TABLE dbo.taking 
  ADD CONSTRAINT FK_taking_classes
  FOREIGN KEY(course_id, classes_id, semester, year) REFERENCES classes(course_id, 
  classes_id, semester, year)

两个表中
课程id
的字符长度不同，导致不创建FK。请在
taking
表中将其设置为varchar（8），该表与
类相同。这肯定会解决问题。我已经使用提供的查询创建了表和FKs。不，这是不正确的。在外键的情况下，允许使用varchars的长度差异。错误消息也非常明确地说明了缺少索引。