学生成绩管理数据库系统有下面有三个关系：

A.select sname，cname，score from student a ，course b ，scores c where a.sno=c.sno and b.cno=c.cno

B.select sname，cname，score from student ，course ，scores

C.select sname，cname，score from student a join course b on a.sno=c.sno join on scores c b.cno=c.cno

D.select sname，cname，score from student a join scores b on a.sno=b.sno join on course c b.cno=c.cno

正确答案：AD