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
平均数：avg
命令：SELECT a.Sid,a.Sname,b.平均数 from ex_xueshengbiao_95 a
left join (SELECT Sid,avg(score) as '平均数' from ex_chengjibiao_95
GROUP BY Sid HAVING avg(score)>=60) b
on a.Sid=b.Sid
