题目：检索" 01 "课程分数小于 60，按分数降序排列的学生信息
分析：按分数降序排列 应该是要用 order by 关键字
降序： desc
升序：asc
我以前的写的想法是： 先查处01 "课程分数小于 60 然后 order by 分数 desc 再去关联student 表
SELECT student.*,r.score FROM student,
(SELECT * FROM student_score WHERE student_score.courseid = 01 AND student_score.score< 60 ORDER BY score DESC)r
WHERE student.id = r.studentid;
其实这样先排序 再去查student 表，查出的学生并不是按照分数降序排序的 没有达到效果
优化：先不排序，查出来学生再排序
SELECT student.*,r.score FROM student,
(SELECT * FROM student_score WHERE student_score.courseid = 01 AND student_score.score< 60)r
WHERE student.id = r.studentid ORDER BY r.score DESC;
再次优化：可以多表查询
SELECT student.*,score FROM student,student_score WHERE
student.id = student_score.studentid AND student_score.courseid = 01 AND student_score.score < 60 ORDER BY score DESC
