mysql - 添加外键约束失败。缺少约束索引错误代码 : 1822

Question

因此，我尝试创建一个表“Precession_to”，它基本上是一个关系，显示哪些类被视为特定类的先决条件。以下是我的 SQL 表缺陷:

CREATE TABLE Class(
infs CHAR(4) NOT NULL,
course_number CHAR(3) NOT NULL,
PRIMARY KEY (infs,course_number));

CREATE TABLE Prerequisite_to(
    infs CHAR(4),
    course_number CHAR(3),
    PRIMARY KEY (infs,course_number),
    FOREIGN KEY (infs) REFERENCES Class(infs),
    FOREIGN KEY (course_number) REFERENCES Class(course_number)
)

但是，当我执行脚本时，出现此错误:

17:29:43 CREATE TABLE Prerequisite_to( infs CHAR(4), course_number CHAR(3), PRIMARY KEY (infs,course_number), FOREIGN KEY (infs) REFERENCES Class(infs), FOREIGN KEY (course_number) REFERENCES Class(course_number) ) Error Code: 1822. Failed to add the foreign key constraint. Missing index for constraint 'prerequisite_to_ibfk_2' in the referenced table 'Class' 0.00038 sec

Answer 1

您有一个复合主键，因此您需要一个复合外键:

CREATE TABLE Prerequisite_to(
    infs CHAR(4),
    course_number CHAR(3),
    PRIMARY KEY (infs,course_number),
    FOREIGN KEY (infs, course_number) REFERENCES Class(infs, course_number)
);

郑重声明一下，我不喜欢复合主键。我还认为先决条件需要两个类(class)引用资料。所以:

CREATE TABLE Classes (
    class_id int auto_increment primary key,
    infs CHAR(4) NOT NULL,
    course_number CHAR(3) NOT NULL,
    unique (infs, course_number)
);

CREATE TABLE Prerequisites (
    preresequisite_id int auto_increment primary key,
    class_id int,
    prerequisite_class_id int,
    FOREIGN KEY (class_id) REFERENCES Classes(class_id),
    FOREIGN KEY (prerequisite_class_id) REFERENCES Classes(class_id)
);