尝试创建具有多个外键的表时收到外键约束错误

Question

CREATE TABLE IF NOT EXISTS 
Student_Clinic_STD_DB.Visit

 (
  `Room_id` INT NOT NULL,


  `P_ID` INT NOT NULL,


  `Dr_ID` INT(4) NOT NULL,

  `Time_Date` DATE NOT NULL,

  `N_ID` INT(6) NULL,

  PRIMARY KEY (`Time_Date`),

  INDEX `Room_Id` (`Room_id` ASC) VISIBLE,

  INDEX `Dr_id_idx` (`Dr_ID` ASC) VISIBLE,

  INDEX `N_Id_idx` (`N_ID` ASC) VISIBLE,

    FOREIGN KEY (`Dr_ID`)

    REFERENCES `Student_Clinic_STD_DB`.`Doctors` (`Dr_ID`));

    **FOREIGN KEY (`N_ID`)
    REFERENCES `Student_Clinic_STD_DB`.`Nurses` (`N_id`)**

ENGINE = InnoDB;

以粗体显示的行是引发错误的原因。该错误表明外来人在此职位无效，需要EOF，Alter等。