2 . 查询平均成绩大于等于 60 分的同学的学生编号和学生姓名和平均成绩

首先查出平均成绩大于60分的sid和平均分：
SELECT AVG(score) AS ss,sid FROM sc GROUP BY sid HAVING AVG(score) > 60
然后连表查询，右链接：
SELECT student.sid ,student.Sname ,av.ss FROM student RIGHT JOIN
(SELECT AVG(score) AS ss,sid FROM sc GROUP BY sid HAVING AVG(score) > 60 )AS av
ON student.SId = av.sid

3 . 查询在 SC 表存在成绩的学生信息

首先查询sid在sc表中：
SELECT sid FROM sc GROUP BY sid

然后再student去重：右链接：

SELECT DISTINCT * FROM student RIGHT JOIN
(SELECT sid FROM sc GROUP BY sid) AS rr
ON student.sid = rr.sid