題目：檢索" 01 "課程分數小於 60，按分數降序排列的學生信息
分析：按分數降序排列 應該是要用 order by 關鍵字
降序： desc
升序：asc
我以前的寫的想法是： 先查處01 "課程分數小於 60 然后 order by 分數 desc 再去關聯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;
其實這樣先排序 再去查student 表，查出的學生並不是按照分數降序排序的 沒有達到效果
優化：先不排序，查出來學生再排序
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;
再次優化：可以多表查詢
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
