CREATE DATABASE test;
use test;
CREATE TABLE classes(
    class_id int primary key auto_increment,
    class_name varchar(20) not null
);
CREATE TABLE students(
    student_id int primary key auto_increment,
    name varchar(50) not null,
    age int,
    class_id int,
    score float,/*decimal函数是表示共有5位数，其中2位是小数*/
    foreign key (class_id) references classes(class_id)
);
create TABLE score_log(
    log_id int primary key auto_increment,
    student_id int,
    old_score float,
    new_score float,
    change_time datetime
);
insert into classes (class_name) values('一班'),('二班'),('三班');
insert into students (name,age,class_id,score) values
 ('张三',13,1,23.1),('李四',14,2,34.3),('王五',34,3,45.6),
 ('张六',23,2,67),('王六',22,3,58.6),('李八',23,1,90);