原文 根据条件合并结果集
第一个查询是将三个表中的列通过键连接起来，并将查询结果重命名为别名aaa；
最后将两个查询的结果集aaa,bbb，通过 aaa.student_id = bbb.studentid连接起来。
select aaa.*,bbb.required,bbb.elective from (select c.student_id,c.chinese_name,c.user_class, sum(decode(b.coursetype,1,a.credit,0)) get_required_total,sum(decode(b.coursetype,0,a.credit,0)) get_elective_total, sum(decode(b.coursetype,1,a.coursecredit,0)) reg_required_total,sum(decode(b.coursetype,0,a.coursecredit,0)) reg_elective_total from sm_course_student a,sm_courses b,sm_students c where a.course_id=b.courseid and a.student_id=c.student_id and c.user_class like ‘10SH%‘ group by c.student_id,c.chinese_name,c.user_class )aaa, (select studentid,required,elective from sm_school_roll where classname like ‘10SH%‘ )bbb where aaa.student_id = bbb.studentid
原文：http://www.cnblogs.com/arxive/p/5959075.html