题目：26：查询平均成绩大于等于 85 的所有学生的学号、姓名和平均成绩
分析：这个应该是根据student 进行分组 group by 再根据 having >= 85 进行过滤，然后在关联student 信息表，拿到学生的基本信息
SELECT student.id, student.stdentname,AVG(student_score.score) AS a FROM student_score, student
WHERE student.id = student_score.studentid
GROUP BY student_score.studentid HAVING a>=85
题目27：查询课程名称为「数学」，且分数低于 60 的学生姓名和分数
分析： 这个应该是多表查询 student , student_course, student_score 三个表查询
SELECT student.id,student_course.coursename, student.stdentname,student_score.score FROM student_course, student_score,student
WHERE student_course.coursename = "数学" AND student_course.id = student_score.courseid AND student_score.score<60
AND student_score.studentid = student.id
