--表tb_user

create table tb_user (

id int not null,

username varchar(32) not null

);

--給tb_user新增主鍵

alter table tb_user add constraint pk_users primary key (id);

--表tb_course

create table tb_course (

id int not null primary key,

name varchar(20) not null

);

--表tb_mark

create table tb_mark (

uid int not null,

cid int not null,

mark int not null,

primary key (uid, cid)

);

--給tb_mark新增外來鍵

alter table tb_mark add constraint fk_uid foreign key(uid) references tb_user(id);

alter table tb_mark add constraint fk_cid foreign key(cid) references tb_course(id);