我是结构化查询语言的初学者。我想添加具有不同外键的多列。例如：

drop schema humman;

create schema humman;

CREATE TABLE humman.father (
id int not null auto_increment,
firstname varchar(200) not null,
primary key(id)
);

create table humman.mather(
id int not null auto_increment,
FirstName varchar(200),
primary key(id)
);

CREATE TABLE humman.child (
id int not null auto_increment,
firstname varchar(200) not null,
primary key(id)
);

use `humman`;

alter table humman.child 
ADD `parentId` int ,
ADD `motherId` int,
ADD  foreign key (`parentId`) references father(`id`),
ADD foreign key (`motherId`) references mother(`id`);

错误代码：1215无法添加外键约束