例3.5建立一个“学生”表student

create table student
    (sno char(9) primary key,
    sname char(20) unique,
    ssex char(2),
    sage smallint,
    sdept char(20)
    );

例3.6建立一个“课程”表course

create table course
    (cno char(4) primary key,
    cname char(40) not null,
    cpno char(4),
    ccredit smallint,
    foreign key(cpno) references course(cno)
    );

例3.7建立学生选课表sc

create table sc
    (sno char(9),
    cno char(4),
    grade smallint,
    primary key(sno,cno),
    foreign key(sno) references student(sno),
    foreign key(cno) references course(cno)
    );