求大佬解答：编写SQL代码查询选修了课程并且选修的课程都不拥有先行课程的学生学号和姓名（用exists）

已知关系模式：

学生表：Student(Sno,Sname,Ssex,Sage,Sdept)

课程表：Course(Cno,Cname,Cpno,Ccredit)

学生选课表：SC(Sno,Cno,Grade)create table student 
 

建立表： 

(sno char(9) primary key,
 sname char(20) unique,
 ssex char(10),
 sage smallint,
 sdept char(20)
 );
 
 create table course
 (cno char(4) primary key,
 cname char(40) not null,
 cpno char(4),
 ccredit smallint,
 foreign key(cpno) references course(cno)
 ); 
 
 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)
 );

插入数据：

insert into student values(20125121,'liyong','male',20,'cs');
 insert into student values(20125122,'liuchen','female',19,'cs');
 insert into student values(20125123,'wangmin','female',18,'ma'); 
 insert into student values(20125125,'zhangli','male',19,'is');
 
 insert into course values(2,'math',null,2);
 insert into course values(6,'data processing',null,2);
 insert into course values(7,'pascal language',6,4);
 insert into course values(4,'operating system',6,3);

  insert into sc values(20125123,6,98);
  insert into sc values(20125125,7,90);
 insert into course values(5,'data structure',7,4);
 insert into course values(1,'database',5,4);
 insert into course values(3,'information systems',1,4);
 insert into sc values(20125121,1,92);
 insert into sc values(20125121,2,85);
 insert into sc values(20125121,3,88);
 insert into sc values(20125122,2,90);
 insert into sc values(20125122,3,80);

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