15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩（重点）

SELECT a.s_id,a.s_name,avg(s_score) FROM student as a
INNER JOIN score as b
ON a.s_id=b.s_id

WHERE a.s_id IN(
SELECT s_id FROM score
WHERE s_score<60 GROUP BY s_id HAVING COUNT(DISTINCT c_id)>=2
  )
GROUP BY a.s_id,s_name