你的 成绩表中 应该要有课程信息,课程要和成绩关联,所以 要在 课程表+成绩表加主外键关联。以下是修改后的表,可以参考
课程表:course 课程名:cname,课程id,
学生表:student 姓名:sname 学号:sno
成绩表:sc 成绩:grade,学号:sno, 课程cId
select student.sno,student.sname,sc.grade from student left join sc on sc.sno = student.sno
where sc.grade > (select sc.grade from student left join sc on sc.sno = student.sno
left join sc.cId= course.id
where course.cname ='经济学' and sname='欧阳宝贝')