高分求SQL优化有关问题

高分求SQL优化问题
有以下表结构：

成绩表(CJB)结构 

列名                          可空值否  类型              备注 
------------------------------- -------- ---- 
KCH                            NOT NULL VARCHAR2(10)      课程号 
KCM                                      VARCHAR2(50)      课程名 
KXH                                      NUMBER(4)          课序号 
XH                              NOT NULL VARCHAR2(12)      学号 
XNXQ                                    VARCHAR2(11)      学年学期 
XF                                      NUMBER(5,1)        学分 
XS                                      NUMBER(4,1)        学时数 
KCSX                                    VARCHAR2(4)        课程属性 
KSCJ                                    NUMBER(5,1)        考试成绩 
CXBKBZ                                  VARCHAR2(4)    成绩补考备注(补考,重修,辅修) 

... 

学籍表(XJB)结构: 

列名                          可空值否  类型            备注 
------------------------------- -------- ---- 
XH                              NOT NULL VARCHAR2(12)    学号 
XM                              NOT NULL VARCHAR2(40)    姓名