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+笔记：
+
+今天写练习，巩固mysql
+
+作业：
+
+```mysql
+CREATE DATABASE ab CHARSET utf8;
+use ab;
+
+# 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);
+
+
+-- 
+-- 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
+	* 
+FROM
+	(
+	SELECT DISTINCT
+		st.student_id,
+		st.student_name,
+		st.gender,
+		st.birthday,
+		c.course_name,
+		sc.score 
+	FROM
+		course c
+		LEFT JOIN score sc ON c.course_id = sc.course_id
+		LEFT JOIN student st ON sc.student_id = st.student_id 
+	WHERE
+		c.course_id = 1 
+		) a,(
+	SELECT DISTINCT
+		st.student_id,
+		st.student_name,
+		st.gender,
+		st.birthday,
+		c.course_name,
+		sc.score 
+	FROM
+		course c
+		LEFT JOIN score sc ON c.course_id = sc.course_id
+		LEFT JOIN student st ON sc.student_id = st.student_id 
+	WHERE
+		c.course_id = 2 
+	) b 
+WHERE
+	a.student_id = b.student_id 
+	AND a.score > b.score;
+
+
+-- 2. 查询同时参与" 1 "课程和" 2 "课程考试的学生信息
+
+SELECT * from student where  student_id in
+(SELECT s.student_id from score s,score s1 WHERE s.student_id=s1.student_id and s.course_id=1 and s1.course_id=2); 
+
+
+
+-- 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 (SELECT * from score WHERE course_id=1) t1 LEFT JOIN (SELECT * from score WHERE course_id=2) t2 ON t1.student_id=t2.student_id;
+-- 4. 查询不存在" 1 "课程但存在" 2 "课程的情况
+SELECT DISTINCT * from score c,score c1 WHERE c.student_id=c1.student_id and c.course_id!=1 and c1.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
+WITH a as
+(SELECT DISTINCT sc.student_id,student_name ,avg(score)over(PARTITION by sc.student_id)c  from score sc left join student s on sc.student_id=s.student_id)
+SELECT * from a WHERE a.c>60;
+
+-- 6. 查询在成绩表存在成绩的学生信息
+SELECT * from student WHERE  student_id in
+(SELECT DISTINCT student_id from score)
+
+-- 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 DISTINCT st.student_id,COUNT(course_id)over(PARTITION by st.student_id),SUM(score) over(PARTITION by st.student_id) from score sc RIGHT JOIN student st on st.student_id=sc.student_id ;
+
+-- 8. 查询「李」姓老师的数量
+SELECT teacher_name,count(*)over() 数量 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 * from student where student_id  in
+(SELECT student_id from score where course_id in
+(SELECT course_id from course WHERE teacher_id in
+(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 DISTINCT student_id from score where student_id not in
+(SELECT s1.student_id from score s1,score s2,score s3 where s1.student_id=s2.student_id and s2.student_id=s3.student_id and s1.course_id=1 and s2.course_id=2 and s3.course_id=3));
+
+-- 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 DISTINCT student_id from score where course_id  in
+(SELECT course_id from score where student_id=1) and student_id !=1);
+
+-- 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 DISTINCT s1.student_id from score s1,score s2,score s3 where s1.student_id=s2.student_id and s2.student_id=s3.student_id and s1.course_id =
+(SELECT course_id from score where student_id=1 LIMIT 0,1) and s2.course_id  = (SELECT course_id from score where student_id=1 LIMIT 1,1)and s3.course_id  = (SELECT course_id from score where student_id=1 LIMIT 2,1)  and s1.student_id !=1 );
+
+
+-- 13. 查询没学过「李白」老师讲授的任一门课程的学生姓名
+SELECT * from student where student_id=
+(SELECT DISTINCT student_id from score where student_id not in 
+(SELECT student_id from score where course_id  in
+(SELECT course_id from course where teacher_id =
+(SELECT teacher_id  from teacher WHERE teacher_name='李白')) GROUP BY student_id));
+
+-- 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
+with a as 
+(SELECT *,avg(score)over(PARTITION by student_id ) av ,COUNT(score)over(PARTITION by student_id ) s  from score where score<60 )
+SELECT DISTINCT st.student_id ,student_name,a.av 平均成绩  from a inner JOIN student st on a.student_id=st.student_id where  a.s>=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 st.*,sc.score from  score sc,student st where sc.student_id=st.student_id and 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  st.student_name, sc.score, AVG(sc.score)over(PARTITION by sc.course_id ORDER BY sc.score DESC) from score sc LEFT JOIN student st on sc.student_id=st.student_id;
+
+
+-- 17. 查询各科成绩最高分、最低分和平均分
+
+
+SELECT course_id, 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 c.course_id, c.course_name, COUNT(DISTINCT sc.student_id) AS num,
+MAX(sc.score) AS max_score, MIN(sc.score) AS min_score, AVG(sc.score) AS avg_score,
+SUM(CASE WHEN sc.score >= 60 THEN 1 ELSE 0 END)/COUNT(sc.score)*100 AS pass_rate,
+SUM(CASE WHEN sc.score >= 70 AND sc.score < 80 THEN 1 ELSE 0 END)/COUNT(sc.score)*100 AS medium_rate,
+SUM(CASE WHEN sc.score >= 80 AND sc.score < 90 THEN 1 ELSE 0 END)/COUNT(sc.score)*100 AS good_rate,
+SUM(CASE WHEN sc.score >= 90 THEN 1 ELSE 0 END)/COUNT(sc.score)*100 AS excellent_rate
+FROM course c
+LEFT JOIN score sc ON c.course_id=sc.course_id
+GROUP BY c.course_id
+ORDER BY num DESC, c.course_id ASC;
+-- 19. 按各科成绩进行排序，并显示排名，Score 重复时保SELECT留名次空缺 
+
+
+SELECT DISTINCT s1.*,rank()over(PARTITION by s1.course_id,s2.student_id ORDER BY s1.score DESC) from score s1,score s2  where  s1.course_id=s2.course_id;
+
+-- 20. 按各科成绩进行排序，并显示排名，Score 重复时合并名次
+SELECT DISTINCT s1.*,dense_rank()over(PARTITION by s1.course_id,s2.student_id ORDER BY s1.score DESC) from score s1,score s2  where  s1.course_id=s2.course_id;
+
+
+-- 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 s.student_id,s.student_name, SUM(sc.score) 总成绩,RANK() over(ORDER BY SUM(sc.score) DESC) 排名 from student s JOIN score sc ON s.student_id = sc.student_id GROUP BY sc.student_id;
+-- 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 s.student_id,s.student_name, SUM(sc.score) 总成绩,DENSE_RANK() over(ORDER BY SUM(sc.score) DESC) 排名 from student s JOIN score sc ON s.student_id = sc.student_id GROUP BY sc.student_id;
+-- 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 
+    c.course_id,
+    c.course_name,
+    COUNT(CASE WHEN sc.score >= 85 AND sc.score <= 100 THEN sc.student_id END) AS score_85_100_count,
+    COUNT(CASE WHEN sc.score >= 70 AND sc.score < 85 THEN sc.student_id END) AS score_70_85_count,
+    COUNT(CASE WHEN sc.score >= 60 AND sc.score < 70 THEN sc.student_id END) AS score_60_70_count,
+    COUNT(CASE WHEN sc.score >= 0 AND sc.score < 60 THEN sc.student_id END) AS score_0_60_count,
+    ROUND(COUNT(CASE WHEN sc.score >= 85 AND sc.score <= 100 THEN sc.student_id END) * 100.0 / COUNT(sc.student_id), 2) AS score_85_100_percentage,
+    ROUND(COUNT(CASE WHEN sc.score >= 70 AND sc.score < 85 THEN sc.student_id END) * 100.0 / COUNT(sc.student_id), 2) AS score_70_85_percentage,
+    ROUND(COUNT(CASE WHEN sc.score >= 60 AND sc.score < 70 THEN sc.student_id END) * 100.0 / COUNT(sc.student_id), 2) AS score_60_70_percentage,
+    ROUND(COUNT(CASE WHEN sc.score >= 0 AND sc.score < 60 THEN sc.student_id END) * 100.0 / COUNT(sc.student_id), 2) AS score_0_60_percentage
+FROM course c
+JOIN score sc ON c.course_id = sc.course_id
+GROUP BY c.course_id, c.course_name;
+
+-- 24. 查询各科成绩前三名的记录
+
+SELECT * from
+(SELECT *,ROW_NUMBER()over(PARTITION by course_id ORDER BY score DESC) a from score sc )b INNER JOIN student st  on b.student_id=st.student_id WHERE b.a in (1,2,3)
+
+
+
+-- 25. 查询每门课程被选修的学生数
+SELECT DISTINCT sc.course_id,course_name,COUNT(student_id)over(PARTITION by course_id) 学生数  from score sc INNER JOIN course co ON sc.course_id= co.course_id
+-- 26. 查询出只选修两门课程的学生学号和姓名
+SELECT  sc.student_id, student_name, COUNT(sc.course_id) from score sc INNER JOIN student st on sc.student_id=st.student_id GROUP BY sc.student_id HAVING COUNT(sc.course_id)=2
+-- 27. 查询男生、女生人数
+SELECT gender, COUNT(gender) 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 DISTINCT student_name,gender,COUNT(student_name)from student  GROUP BY student_name,gender HAVING COUNT(student_name)>1  ;
+
+-- 30. 查询 1990 年出生的学生名单
+SELECT  * from student WHERE birthday like '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 course_id, AVG(score) 平均成绩 FROM score GROUP BY course_id ORDER BY 平均成绩 DESC, course_id ASC;
+
+-- 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  sc.student_id, student_name,AVG(score)  from score sc INNER JOIN student st on sc.student_id=st.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 sc.course_id ,student_name,score from course co INNER JOIN score sc on co.course_id=sc.course_id INNER JOIN student st on sc.student_id=st.student_id  where 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 * from student st LEFT JOIN score sc ON st.student_id = sc.student_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,co.course_name,score from course co INNER JOIN score sc on co.course_id=sc.course_id INNER JOIN student st on sc.student_id=st.student_id  where score>70;
+
+-- 36. 查询不及格的课程
+SELECT co.course_id,co.course_name,score from course co INNER JOIN score sc on co.course_id=sc.course_id  where 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 sc.course_id,st.student_name,score from  score sc INNER JOIN student st on sc.student_id=st.student_id  where  sc.course_id=1 and score>80;
+
+-- 38. 求每门课程的学生人数
+SELECT DISTINCT co.course_id,co.course_name,COUNT(student_id)over(PARTITION by course_name) from course co INNER JOIN score sc on co.course_id=sc.course_id ;
+ 
+-- 39. 成绩不重复，查询选修「李白」老师所授课程的学生中，成绩最高的学生信息及其成绩
+
+
+
+SELECT st.*,MAX(score) a  from course co INNER JOIN score sc on co.course_id=sc.course_id INNER JOIN student st on sc.student_id=st.student_id INNER JOIN teacher t on co.teacher_id=t.teacher_id  where teacher_name= '李白' GROUP BY sc.student_id  ORDER BY a desc LIMIT 0,1;
+
+
+
+-- 40. 成绩有重复的情况下，查询选修「李白」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.student_id, student.student_name, score.score FROM 
+(SELECT course_id, MAX(score) AS max_score FROM score
+WHERE course_id IN (SELECT course_id FROM course WHERE teacher_id = (SELECT teacher_id FROM teacher WHERE teacher_name = '李白'))
+GROUP BY course_id) AS max_scores
+JOIN score ON max_scores.course_id = score.course_id AND max_scores.max_score = score.score
+JOIN student ON score.student_id = student.student_id;
+
+-- 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 JOIN score s2 ON 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 course_id, student_id, score FROM
+(SELECT *,RANK() over(partition by course_id ORDER BY score DESC) a FROM score) b
+WHERE a <= 2;
+
+-- 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 course_id, COUNT(*) AS student_count
+FROM score
+GROUP BY course_id
+HAVING COUNT(*) > 5;
+
+-- 44. 检索至少选修两门课程的学生学号
+SELECT student_id
+FROM score
+GROUP BY student_id
+HAVING COUNT(*) >= 2;
+
+-- 45. 查询选修了全部课程的学生信息
+SELECT s.student_id, s.student_name
+FROM student s
+JOIN (
+    SELECT student_id, COUNT(DISTINCT course_id) AS course_count
+    FROM score
+    GROUP BY student_id
+) t ON s.student_id = t.student_id
+JOIN (
+    SELECT COUNT(*) AS total_courses
+    FROM course
+) c ON t.course_count = c.total_courses;
+
+-- 46. 查询各学生的年龄，只按年份来算
+SELECT student_id, YEAR(CURRENT_DATE()) - YEAR(birthday) AS age
+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_id,
+       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 student_id, student_name, birthday
+FROM student
+WHERE WEEK(birthday) = WEEK(CURRENT_DATE());
+
+-- 49. 查询下周过生日的学生
+SELECT student_id, student_name, birthday
+FROM student
+WHERE WEEK(birthday) = WEEK(CURRENT_DATE()) + 1;
+ 
+-- 50. 查询本月过生日的学生
+SELECT student_id, student_name, birthday
+FROM student
+WHERE MONTH(birthday) = MONTH(CURRENT_DATE());
+
+-- 51. 查询下月过生日的学生
+SELECT student_id, student_name, birthday
+FROM student
+WHERE MONTH(birthday) = MONTH(CURRENT_DATE()) + 1;
+
+```
+