sql exsits 用法

--创建学生表
create table student(sno varchar2(20),sname varchar2(20));
--创建课程表
create table course(cno varchar2(20),cname varchar2(20));
--创建学生课程关联表
create table sc(sno varchar2(20),cno varchar2(20));

--初始化学生表数据
insert into student values('200215121','李勇');
insert into student values('200215122','刘晨');
insert into student values('200215123','王敏');
insert into student values('200215124','张立');

--初始化课程表数据
insert into course values('1','数据库');
insert into course values('2','数学');
insert into course values('3','信息系统');
insert into course values('4','操作系统');
insert into course values('5','数据结构');
insert into course values('6','数据处理');
insert into course values('7','PASCAL语言');

--初始化学生，课程表数据
insert into sc values('200215121','1');
insert into sc values('200215121','2');
insert into sc values('200215121','3');
insert into sc values('200215121','4');
insert into sc values('200215121','5');
insert into sc values('200215121','6');
insert into sc values('200215121','7');
insert into sc values('200215121','2');
insert into sc values('200215121','3');
insert into sc values('200215122','4');
insert into sc values('200215122','5');

--查询选修了课程号为1的学生姓名
--方法一：
select sno from student
  where
     exists
       (select * from sc where student.sno = sno and cno = '1')
--方法二：
select student.sno
from student,sc
where student.sno = sc.sno and sc.cno = '1';

--方法三
select  student.sname from student
where student.sno in
     (select sc.sno from sc where sc.cno = '1');

--查询没有选修课程1的学生姓名
select sname from student
     where not exists(select * from sc where student.sno = sc.sno and sc.cno = '1')
     
--查询选修全部课程的学今生姓名     
--方法一：
select Sname
from Student
where not exists
          (
                 select *
                 from Course
                 where not exists
                           (
                                 select *
                                 from  SC
                                 where Sno=Student.Sno AND
                                            Cno=Course.Cno
                           ) );
--方法二：                           
select student.sno
from student  where student.sno in
(select sc.sno from sc group by sc.sno having count(*) = (select count(*) from course));