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+~~~mysql
+create database p20 charset utf8;
+use p20;
+
+# 学生表
+CREATE TABLE IF NOT EXISTS `student`(
+    `student_id` INT(10) NOT NULL AUTO_INCREMENT COMMENT '学号',
+    `student_name` VARCHAR(10) NOT NULL DEFAULT '匿名' COMMENT '姓名',
+    `birthday` DATETIME NOT NULL COMMENT '出生日期',
+    `gender` VARCHAR(10) NOT NULL DEFAULT '男' COMMENT '性别',
+    PRIMARY KEY(`student_id`)
+)ENGINE=INNODB CHARSET=utf8;
+
+INSERT INTO `student` VALUES
+(1 , '赵雷' , '1990-01-01' , '男'),
+(2 , '钱电' , '1990-12-21' , '男'),
+(3 , '孙风' , '1990-12-20' , '男'),
+(4 , '李云' , '1990-12-06' , '男'),
+(5 , '周梅' , '1991-12-01' , '女'),
+(6 , '吴兰' , '1992-01-01' , '女'),
+(7 , '郑竹' , '1989-01-01' , '女'),
+(8 , '张三' , '2017-12-20' , '女'),
+(9 , '李四' , '2017-12-25' , '女'),
+(10 , '李四' , '2012-06-06' , '女'),
+(11 , '赵六' , '2013-06-13' , '女'),
+(12 , '孙七' , '2014-06-01' , '女');
+-- 课程表
+CREATE TABLE IF NOT EXISTS `course`(
+    `course_id` INT(4) NOT NULL AUTO_INCREMENT COMMENT '课程编号',
+    `course_name` VARCHAR(10) NOT NULL COMMENT '课程名',
+    `teacher_id` INT(10) NOT NULL COMMENT '任课教师工号',
+    PRIMARY KEY(`course_id`)
+)ENGINE=INNODB CHARSET=utf8;
+ 
+INSERT INTO `course` VALUES
+(1, '语文', 2),
+(2, '数学', 1),
+(3, '英语', 3);
+# 教师表
+CREATE TABLE IF NOT EXISTS `teacher`(
+    `teacher_id` INT(10) NOT NULL AUTO_INCREMENT COMMENT '教师工号',
+    `teacher_name` VARCHAR(10) NOT NULL DEFAULT '匿名' COMMENT '教师姓名',
+    PRIMARY KEY(`teacher_id`)
+)ENGINE=INNODB CHARSET=utf8;
+ 
+INSERT INTO `teacher` VALUES
+(1, '高斯'),
+(2, '李白'),
+(3, 'Trump');
+
+-- 成绩表
+CREATE TABLE IF NOT EXISTS `score`(
+    `student_id` INT(10) NOT NULL COMMENT '学号',
+    `course_id` INT(4) NOT NULL COMMENT '课程编号',
+    `score` DECIMAL(18,1) COMMENT '成绩',
+    KEY(`course_id`)
+)ENGINE=INNODB CHARSET=utf8;
+ 
+INSERT INTO `score` VALUES
+(1 , 1 , 80),
+(1 , 2 , 90),
+(1 , 3 , 99),
+(2 , 1 , 70),
+(2 , 2 , 60),
+(2 , 3 , 80),
+(3 , 1 , 80),
+(3 , 2 , 80),
+(3 , 3 , 80),
+(4 , 1 , 50),
+(4 , 2 , 30),
+(4 , 3 , 20),
+(5 , 1 , 76),
+(5 , 2 , 87),
+(6 , 1 , 31),
+(6 , 3 , 34),
+(7 , 2 , 89),
+(7 , 3 , 98);
+
+
+
+
+## 题目：
+
+-- 1. 查询" 1 "课程比" 2 "课程成绩高的学生的信息（学号、姓名、性别、出生日期）及课程分数
+select student.*,a.x 课程1成绩,b.y 课程2成绩
+   from (select score.student_id,score as x
+   from score where  course_id=1) a,(select score.student_id,score as y
+   from score where  course_id=2) b,student where a.student_id=b.student_id and a.student_id=student.student_id and a.x>b.y;
+	 
+	 
+-- 2. 查询同时参与" 1 "课程和" 2 "课程考试的学生信息
+select * from (select * from score where course_id=1) a,(select * from score where course_id=2) b where a.student_id=b.student_id;
+select * from student s , (select a.student_id from (select * from score where course_id=1) a,(select * from score where course_id=2) b where a.student_id=b.student_id) d where s.student_id=d.student_id;
+
+
+# 3. 查询存在" 1 "课程但可能不存在" 2 "课程的情况(不存在时显示为 null )
+   select *
+   from course left join(select *
+   from score where student_id not in(select student_id from score where course_id=2)) b
+       on b.course_id=course.course_id;
+   
+   
+   # 4. 查询不存在" 1 "课程但存在" 2 "课程的情况
+   select *
+   from score where student_id not in(select student_id from score where course_id=1) and course_id=2;
+   
+   
+   # 5. 查询平均成绩大于等于 60 分的同学的学生编号、姓名和平均成绩
+   select student.*,a.平均成绩
+   from student,(select avg(score) 平均成绩,avg(score)>60 成绩,student_id from score group by student_id having 成绩=1)a where a.student_id=student.student_id;
+
+
+-- 6. 查询在成绩表存在成绩的学生信息
+select distinct b.* from score a inner join student b on a.student_id=b.student_id where score is not null;
+
+-- 7. 查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩(没成绩的显示为 null )
+select a.student_id,a.student_name,count(course_id),sum(score) from student a,score b where a.student_id=b.student_id GROUP BY b.student_id;
+
+-- 8. 查询「李」姓老师的数量
+select count(*) from teacher where teacher_name like '李%';
+
+
+-- 9. 查询学过「李白」老师授课的同学的信息
+select b.* from score a inner join student b on a.student_id=b.student_id where a.course_id=(select course_id from course where teacher_id=(select teacher_id from teacher where teacher_name='李白'));
+
+
+-- 10. 查询没有学全所有课程的同学的信息
+select * from student where student_id in (select student_id from score group by student_id having count(*)<(select count(*) from course));
+
+
+-- 11. 查询至少有一门课与学号为" 1 "的同学所学相同的同学的信息
+select * from student where student_id in (select student_id from score where course_id = any (select course_id from score where student_id=1) group by student_id);
+
+
+
+-- 12. 查询和" 1 "号的同学学习的课程完全相同的其他同学的信息
+select * from student where student_id in (select student_id from score where course_id = all (select course_id from score where student_id=1) group by student_id);
+
+
+# 13. 查询没学过「李白」老师讲授的任一门课程的学生姓名
+   select distinct student.student_id,student_name
+   from student left join score s on student.student_id = s.student_id
+   where student.student_id not in
+         (select student_id
+          from score,
+               course,
+               teacher
+          where score.course_id = course.course_id
+            and course.teacher_id = teacher.teacher_id
+          and teacher_name='李白');
+   
+   # 14. 查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
+   select student.student_id,student_name,avg(score)
+   from score,student where score.student_id=student.student_id and score<60 group by student_id having count(score.student_id)>=2;
+   
+   # 15. 检索" 1 "课程分数小于 60，按分数降序排列的学生信息
+   select *
+   from score where course_id=1 and score<60 order by score desc ;
+-- 16. 按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
+select student_id 学生id,score 课程成绩,avg(score) over (partition by student_id) 平均成绩 from score;
+
+
+-- 17. 查询各科成绩最高分、最低分和平均分
+select max(score),min(score),avg(score) from score group by course_id;
+
+-- 18. 以如下形式显示：课程 ID，课程 name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率（及格为>=60，中等为：70-80，优良为：80-90，优秀为：>=90），要求输出课程号和选修人数，查询结果按人数降序排列，若人数相同，按课程号升序排列
+SELECT
+	course_id 课程ID,(select course_name from course where course_id=s.course_id) 课程name,max(score) 最高分, min(score) 最低分,avg(score) 平均分,count(*) 选修人数,
+	count(score>=90 or null)/count(*) 优秀率,
+	count(score>=80 and score<90 or null)/count(*) 优良率,
+	count(score>=70 and score<80 or null)/count(*) 中等率,
+	count(score>=60 or null)/count(*) 及格率
+	
+FROM
+	score s group by course_id;
+
+
+
+
+-- 19. 按各科成绩进行排序，并显示排名，Score 重复时保留名次空缺
+select *,rank() over (partition by course_id order by score) 各科成绩排名 from score;
+
+
+-- 20. 按各科成绩进行排序，并显示排名，Score 重复时合并名次
+select *,dense_rank() over (partition by course_id order by score) 各科成绩排名 from score;
+
+
+-- 21. 查询学生的总成绩，并进行排名，总分重复时保留名次空缺
+select 总成绩,rank() over (order by 总成绩) from (select sum(score) 总成绩 from score group by student_id) a;
+
+-- 22. 查询学生的总成绩，并进行排名，总分重复时不保留名次空缺
+select 总成绩,dense_rank() over (order by 总成绩) from (select sum(score) 总成绩 from score group by student_id) a;
+
+
+-- 23. 统计各科成绩各分数段人数：课程编号，课程名称，[100-85]，[85-70]，[70-60]，[60-0] 及所占百分比
+SELECT
+	course.course_id 课程编号,course.course_name 课程名称,count(student_id),
+	(SUM(case when score>=85 and score<100 then 1 else 0 end))/count(*)`[100-85]`,
+	(SUM(case when score>=70 and score<85  then 1 else 0 end))/count(*)`[85-70]`,
+	(SUM(case when score>=60 and score<70  then 1 else 0 end))/count(*)`[70-60]`,
+	(SUM(case when score>=0  and score<60  then 1 else 0 end))/count(*)`[60-0]`
+FROM
+	score,
+	course 
+WHERE
+	score.course_id = course.course_id group by course.course_id;
+
+
+
+-- 24. 查询各科成绩前三名的记录
+select * from (select *,dense_rank() over (partition by course_id order by score) `成绩排名` from score) b where `成绩排名`<=3;
+
+
+-- 25. 查询每门课程被选修的学生数
+select course_id,count(student_id) from score group by course_id;
+
+
+-- 26. 查询出只选修两门课程的学生学号和姓名
+select student.*,count(course_id) 选修课程数 from score,student where score.student_id=student.student_id group by student_id having count(course_id)=2;
+
+
+-- 27. 查询男生、女生人数
+select gender,count(*) from student group by gender;
+
+
+-- 28. 查询名字中含有「风」字的学生信息
+select * from student where student_name like '%风%';
+
+-- 29. 查询同名同性学生名单，并统计同名人数
+select student_name,gender,count(student_id) from student group by student_name,gender having count(student_id)>=2;
+
+
+-- 30. 查询 1990 年出生的学生名单
+select * from student where year(birthday)=1990;
+
+
+-- 31. 查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
+select avg(score) from score group by course_id order by course_id desc;
+
+
+-- 32. 查询平均成绩大于等于 85 的所有学生的学号、姓名和平均成绩
+select student.*,avg(score) from score,student where score.student_id=student.student_id group by student_id having avg(score)>=85;
+
+-- 33. 查询课程名称为「数学」，且分数低于 60 的学生姓名和分数
+select student_name,score from score,student where score.student_id=student.student_id and course_id=(select c.course_id from course c where c.course_name='数学'
+) and score<60;
+
+
+-- 34. 查询所有学生的课程及分数情况（存在学生没成绩，没选课的情况）
+select student_name,course_name,score from score inner join student on score.student_id=student.student_id inner join course on score.course_id=course.course_id;
+
+
+-- 35. 查询任何一门课程成绩在 70 分以上的姓名、课程名称和分数
+select student_name,course_name,score from score inner join student on score.student_id=student.student_id inner join course on score.course_id=course.course_id  where score>70;
+
+
+-- 36. 查询不及格的课程
+select distinct course_name from score,course where score.course_id=course.course_id and score<60;
+
+-- 37. 查询课程编号为 1 且课程成绩在 80 分以上的学生的学号和姓名
+select student.student_id,student_name,score from score inner join student on score.student_id=student.student_id where course_id=1 and score>80;;
+
+
+-- 38. 求每门课程的学生人数
+select course_name,count(*) from score inner join course on score.course_id=course.course_id group by course.course_name;
+
+
+
+
+-- 39. 成绩不重复，查询选修「张三」老师所授课程的学生中，成绩最高的学生信息及其成绩
+select student.*,score
+   from course,score,student,teacher where score.course_id=course.course_id
+   and score.student_id=student.student_id
+   and course.teacher_id=teacher.teacher_id
+   and teacher_name='李白'
+   and score=(select max(score) from score,teacher,course where course.teacher_id=teacher.teacher_id and score.course_id=course.course_id and teacher_name='李白')
+   limit 0,1;
+
+
+-- 40. 成绩有重复的情况下，查询选修「张三」老师所授课程的学生中，成绩最高的学生信息及其成绩
+select student.*,score
+   from course,score,student,teacher where score.course_id=course.course_id
+   and score.student_id=student.student_id
+   and course.teacher_id=teacher.teacher_id
+   and teacher_name='李白'
+   and score=(select max(score) from score,teacher,course where course.teacher_id=teacher.teacher_id and score.course_id=course.course_id and teacher_name='李白');
+
+
+
+-- 41. 查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
+select distinct s1.student_id,s1.course_id,s1.score
+   from score s1,score s2 where s1.student_id=s2.student_id
+   and s1.score=s2.score
+   and s1.course_id!=s2.course_id;
+
+
+-- 42. 查询每门功成绩最好的前两名
+select dense_rank() over (partition by course_id order by score) from score ; 
+select * from (select score.course_id,dense_rank() over (partition by course_id order by score) `成绩排名` from score) a,course where a.course_id=course.course_id and `成绩排名`<3;
+
+-- 43. 统计每门课程的学生选修人数（超过 5 人的课程才统计）。
+select score.course_id,course.course_name,count(student_id) from score inner join course on score.course_id=course.course_id  group by score.course_id having count(student_id)>5;
+
+ # 44. 检索至少选修两门课程的学生学号
+select student.*, count(score.student_id)选课数量
+from score,student,course where score.student_id=student.student_id and score.course_id=course.course_id group by student_id having 选课数量>=2;
+
+# 45. 查询选修了全部课程的学生信息
+select student.*, count(score.student_id)选课数量
+from score,student,course where score.student_id=student.student_id and score.course_id=course.course_id group by student_id having 选课数量=3;
+
+# 46. 查询各学生的年龄，只按年份来算
+select student_name,year(now())-year(birthday) 年龄
+from student;
+
+# 47. 按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一
+select student_name,
+    year(current_date) - year(birthday) - (date_format(current_date, '%m%d') < date_format(birthday, '%m%d')) AS age
+from student;
+
+# 48. 查询本周过生日的学生
+select *
+from student where week(birthday)=week(current_date);
+
+# 49. 查询下周过生日的学生
+select *
+from student where week(birthday)=week(current_date)+1;
+
+# 50. 查询本月过生日的学生
+select *
+from student where month(birthday)=month(now());
+
+# 51. 查询下月过生日的学生
+select *
+from student where month(birthday)=month(now())+1;
+~~~
+