mysql - 无法添加外键约束mysql

Question

有谁知道为什么我在这个查询中总是收到这个错误。我很难过:

错误代码:1215。无法添加外键约束

我的代码:

CREATE TABLE countries (
  id INT NOT NULL AUTO_INCREMENT,
  cname VARCHAR(45) NOT NULL,
  PRIMARY KEY (id)
) COMMENT='Country List';

CREATE TABLE members (
  id INT NOT NULL AUTO_INCREMENT,
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  Salt VARCHAR(45),
  PRIMARY KEY (id),
  FOREIGN KEY (Salt) REFERENCES countries(cname)
) COMMENT='stuff';

Answer 1

外键必须引用主键或唯一键。例如，您可以使 countries.cname 独一无二:

CREATE TABLE countries (
  id INT NOT NULL AUTO_INCREMENT,
  cname VARCHAR(45) NOT NULL,
  PRIMARY KEY (id),
  UNIQUE (cname) -- Here
) COMMENT='Country List';

或者，您可以删除 salt 列并通过 id 进行引用:

CREATE TABLE members (
  id INT NOT NULL AUTO_INCREMENT,
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  Salt_id INT, -- Here
  PRIMARY KEY (id),
  FOREIGN KEY (Salt_Id) REFERENCES countries(id) -- And here
) COMMENT='stuff';