1. 创建表的同时添加外键

create table score(
    score int(3),
    st_id int(16),
    cs_id int(16),
    primary key(st_id,cs_id),
    FOREIGN KEY (st_id) REFERENCES student(id),
    FOREIGN KEY (cs_id) REFERENCES classes(id)
);

2. 已经创建表了怎么办：在表的定义外进行添加

alter table 表名 add constraint FK_ID foreign key(你的外键字段名) REFERENCES 外表表名(对应的表的主键字段名);

3.直接在属性值后面添加

create table score(
    cscore int(11),
    st_id int(50) references student(id),
    cs_id int(30) references classes(id),
    primary key(st_id,cs_id)
);

4.添加约束

create table score(
    cscore int(11),
    st_id int(50),
    cs_id int(30),
    primary key(st_id,cs_id),
    CONSTRAINT `FK_ID_ST` FOREIGN KEY (st_id) REFERENCES student(id),
    CONSTRAINT `FK_ID_CS` FOREIGN KEY (cs_id) REFERENCES classes(id)
);

参考：sql增删改查基本语句 - 百度文库