46.#查詢全部學生都選修的課程的課程號和課程名；
select sc.c,max(course.cname),count(sc.s) from sc join course on(sc.c=course.c)
group by sc.c
having count(sc.score) >(select count(distinct s) from sc)

43.查詢每門課程成績最好的前兩名；

SELECT * FROM sc AS a join student on(a.s=student.s)

WHERE 2 > ( SELECT COUNT(*) FROM sc WHERE c = a .c AND score>a .score )
ORDER BY a.score DESC;

40）查詢選修“楊豔”老師所授課程的學生中，成績最高的學生姓名及其成績；

SELECT student.sname,sc.s,sc.score from sc,student,teacher,course
where sc.score =( select max(a.score) from sc a where a.c=sc.c) and teacher.tname='楊豔' and sc.s = student.s and sc.c=course.c and course.t=teacher.t

1/期間玩手機過程，觀看其他網站時間過長，需要控制

2/題目多想想，不要一老看答案，理解完了才算掌握。