创建主表和子表并建立约束

#创建年级表(包含ID和姓名)
CREATE TABLE IF NOT EXISTS grade(
	gradeId INT(4) NOT NULL PRIMARY KEY AUTO_INCREMENT,
	gradeName VARCHAR(50) NOT NULL
)

#创建学生表(包含学号、姓名、年龄、年级、电话、性别、家庭地址)
CREATE TABLE IF NOT EXISTS student(
	studentNo INT(4) NOT NULL PRIMARY KEY AUTO_INCREMENT,
	studentName VARCHAR(20) NOT NULL DEFAULT '匿名',
	age INT(4),
	gradeId INT(4),
	phoneNum VARCHAR(11),
	sex VARCHAR(10),
	address VARCHAR(50),
	CONSTRAINT FK_gradeId FOREIGN KEY(gradeId) REFERENCES grade(gradeId)
 )