create table students(

    sid int not null primary key auto_increment,

    sname char(4) not null,

    ssex char(2) not null

);

create table course(

    cid int not null primary key auto_increment,

    cname varchar(64) not null,

);

create table student_course(

    sid int not null,

    cid int not null,

    constraint for_key_sid foreign key(sid) references students(sid),

    constraint for_key_cid foreign key(cid) references course(cid),

    primary key(sid,cid)

);

查询选修了所有课程的学生的学号和姓名：

select sid,sname from students where not exists(

    select sid from course where not exists(

        select * from student_course where sid = students.sid and cid = course.cid

    )

);