数据库子查询有关问题。请各位大牛赐教_Oracle技术

数据库子查询问题。请各位大牛赐教
S表 (SNO,SNAME) 学生关系。SNO 为学号，SNAME 为姓名
C表 (CNO,CNAME,CTEACHER) 课程关系。CNO 为课程号，CNAME 为课程名，CTEACHER 为任课教师
SC表 (SNO,CNO,SCGRADE) 选课关系表。SCGRADE 为成绩

问题：列出“1”号课成绩比“2”号课成绩高的所有学生的学号

大牛请赐教。

------解决方案--------------------

SQL code

CREATE TABLE s(SNO VARCHAR(10),SNAME VARCHAR2(20));CREATE TABLE c(CNO VARCHAR(10),CNAME VARCHAR2(20),CTEACHER VARCHAR2(10));CREATE TABLE sc(SNO VARCHAR(10),CNO VARCHAR(10),SCGRADE NUMBER(4));INSERT INTO s VALUES ('s001','jack');INSERT INTO s VALUES ('s002','rose');INSERT INTO s VALUES ('s003','mike');INSERT INTO c VALUES ('1','history','t01');INSERT INTO c VALUES ('2','English','t02');INSERT INTO sc VALUES ('s001','1',90);INSERT INTO sc VALUES ('s001','2',80);INSERT INTO sc VALUES ('s002','1',85);INSERT INTO sc VALUES ('s002','2',90);INSERT INTO sc VALUES ('s003','1',80);INSERT INTO sc VALUES ('s003','2',60);SELECT sno FROM SC t WHERE t.cno='1' AND EXISTS (SELECT 1 FROM SC WHERE sno=t.sno AND cno='2' AND scgrade<t.scgrade);输出：SNOs001s003
------解决方案--------------------
select sno from (select t.SNO,t.SCGRADE-nvl(d.SCGRADE ) SCGRADE_c from (SELECT sc.sno, SCGRADE  FROM sc where CNO=1) t,(SELECT sc.sno, SCGRADE  FROM sc where CNO=2) d
where t.sno = d.sno(+)) t where SCGRADE_c>0