mysql增加外键有哪些方法

CREATE TABLE class(cid INT PRIMARY KEY AUTO_INCREMENT,
sname VARCHAR(15) NOT NULL)
INSERT INTO class VALUES(NULL,'六年级一班')
INSERT INTO class VALUES(NULL,'六年级二班')

CREATE TABLE student(sid INT AUTO_INCREMENT,
sname VARCHAR(10) NOT NULL,
s_cid INT REFERENCES class(cid),
PRIMARY KEY(sid)
);

INSERT INTO student VALUES(NULL,'王大锤',2)
INSERT INTO student VALUES(NULL,'胡汉三',3)　　　　//约束不生效，原因待查
INSERT INTO student VALUES(NULL,'卧槽',4)　　　　　//不敢相信，在测试一次

CREATE TABLE student(
sid INT AUTO_INCREMENT,
sname VARCHAR(10),
s_cid INT,
PRIMARY KEY(sid),
FOREIGN KEY (s_cid) REFERENCES class(cid)
);

INSERT INTO student VALUES(NULL,'王大锤',2)
INSERT INTO student VALUES(NULL,'胡汉三',3)　　　//约束生效　　插入失败

CREATE TABLE student(sid INT AUTO_INCREMENT,
sname VARCHAR(10) NOT NULL,
s_cid INT ,
PRIMARY KEY(sid),
CONSTRAINT fk_student_class FOREIGN KEY(s_cid) REFERENCES class(cid)
);

INSERT INTO student VALUES(NULL,'王大锤',2)
INSERT INTO student VALUES(NULL,'胡汉三',3)　　//约束生效　　插入失败

CREATE TABLE student(sid INT AUTO_INCREMENT,
sname VARCHAR(10) NOT NULL,
s_cid INT ,
PRIMARY KEY(sid)
);

ALTER TABLE student ADD FOREIGN KEY (s_cid) REFERENCES class(cid);

INSERT INTO student VALUES(NULL,'王大锤',2)
INSERT INTO student VALUES(NULL,'胡汉三',3)　　//约束生效 　　插入失败