13.5、t_student和t_classes完整示例 ``` drop table if exists t_classes; create table t_classes( classes_id int (3), classes_name varchar(30) not null, constraint pk_classes_id primary key(classes_id) ) drop table if exists t_student; create table t_student( student_id int(10), student_name varchar(50) not null, sex char(2) not null, birthday date not null, email varchar(30) unique, classes_id int (3) not null, constraint pk_student_id primary key(student_id), constraint fk_classes_id foreign key(classes_id) references t_classes(classes_id) ) ```