求最高成绩和最低成绩的学生的信息
select stu.stuid,stu.stuname,c.classname,co.coursename
from students stu
left join classes c on stu.classid=c.classid and c.classname=‘某个班级‘
left join course co on stu.courseid=co.courseid and co.coursename=‘某个科目‘
where max(sc.score) or min(sc.score)
原文：http://www.cnblogs.com/puzhichao/p/7366335.html