mysql4种添加外键方式

首先创建班级表和学生表

​班级表：​

CREATE TABLE class(
c_id INT PRIMARY KEY AUTO_INCREMENT,

s_name VARCHAR(15) NOT NULL)

);

​学生表：​

CREATE TABLE student(
s_id INT AUTO_INCREMENT,

s_name VARCHAR(10) NOT NULL,

PRIMARY KEY(sid)

);

​第一种：在属性后面直接添加​

CREATE TABLE student(sid INT AUTO_INCREMENT,

s_name VARCHAR(10) NOT NULL,

s_cid INT REFERENCES class(c_id),

PRIMARY KEY(s_id)

);

​第二种：使用fk添加​

CREATE TABLE student(sid INT AUTO_INCREMENT,

s_name VARCHAR(10) NOT NULL,

s_cid INT ,

PRIMARY KEY(s_id),

CONSTRAINT fk_student_class FOREIGN KEY(s_cid) REFERENCES class(c_id)

);

第三种：在sql语句未端添加

CREATE TABLE student(

s_id INT AUTO_INCREMENT,

s_name VARCHAR(10),

PRIMARY KEY(s_id),

FOREIGN KEY (s_id) REFERENCES class(c_id)

);

​第四种：在建表后添加外键​

CREATE TABLE student(sid INT AUTO_INCREMENT,

s_name VARCHAR(10) NOT NULL,

s_cid INT ,

PRIMARY KEY(s_id)

);

​注意：class是班级表哈！​