SQL学习（七）：怎么给一张表添加外键（四种方法）_sql为表中添加外键

如何添加外键

方法一：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)
);

方法二：

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

方法三：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)·
);

方法四：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);

参考:
https://www.cnblogs.com/qisong178878915/p/4435488.html