聚集函数
不能出现在where中
所以查出某些同学的数学分数大于平均分15分
select t1.namefrom student t1, course t2, student_course t3where t1.id = t3.student_id and t2.id = t3.course_id and t2.curse_name = 'Math' and t3.score > 15 + ( select avg(t3.score) from course t2, student_course t3 where t2.id = t3.course_id and t2.curse_name = 'Math' )
