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) )