选课系统中的选课表(sc)的主码是()。 A: sid B: cid C: grade D: sid+cid

按课程号分类，统计相应的选课人数（） A: select cid, count(distinct sid) from sc group by cid B: select cid, count(*) from student group by cid C: select cid, count(distinct cid) from sc group by cid D: select cid, count(distinct sid) from student group by cid

按照学号进行分组，统计学生的平均成绩（） A: select sid, avg(grade) from student group by cid B: select sid, avg(grade) from student group by sid C: select sid, avg(grade) from sc group by sid D: select sid, avg(grade) from sc group by cid

查询学生的学号，姓名，选修课程号和课程成绩 A: select sid,sname,cid,grade from student,sc where student.sid=sc.sid B: select * from student,sc where student.sid=sc.sid C: select sid,sname,cid,cname from student,course D: select sid,sname,cid,cname from student,sc

按照学号进行分组，统计学生的平均成绩，且平均分按降序排列 A: select sid, avg(grade) from sc group by sid order by avg(grade) desc B: select sid, avg(grade) from sc group by cid order by avg(grade) desc C: select sid, avg(grade) from student group by sid order by avg(grade) desc D: select sid, avg(grade) from student group by cid order by avg(grade)

查询选修了2门课程以上的学生学号（） A: select sid,count(*) from sc group by sid having count(*)>=2 B: select sid,count(*) from sc group by sid where count(*)>=2 C: select sid,count(*) from sc where count(*)>=2 group by sid D: select sid,count(*) from sc group by cid having count(*)>=2

【单选题】已知Sid商店，Pid商品编号，Cid商品名称，Did经营部，Mgr经营部经理，Scale经营部规模。下列属于3NF的关系模式是_________

查询选修了a001号课程的学生的最高成绩（） A: select cid, max(grade) from course where cid=’a001’ group by cid B: select cid,max(grade) from student where cid=’a001’ group by cid C: select cid, max(grade) from sc where cid=’a001’ group by cid D: select cid, max(grade) from student group by cid having cid=’a001’

统计选修了a001课程的学生的平均成绩( ) A: select cid,avg(grade) from student group by cid having cid=’a001’ B: select cid,avg(grade) from student where cid=’a001’ group by cid C: select cid,avg(grade) from course where cid=’a001’ group by cid D: select cid,avg(grade) from sc where cid=’a001’ group by cid

学生表student中包含字段：学生编号sid（主键），学生姓名sname。成绩表score包含字段：课程编号cid，学生编号sid（外键），学生成绩degree；其中学生表包含10行数据，成绩表包含6行记录（且sid列没有重复值），那么执行SQL语句：select * from student left outer join score on student.sid=score.sid，将返回（）条记录