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+````mysql
+
+
+```mysql
+# MySQL经典50题
+create database tt charset utf8;
+use tt;
+# 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 c.*,a.score,b.score from 
+	(select * from score where course_id = 1) a join 
+	(select * from score where course_id = 2) b on a.student_id = b.student_id join 
+	student c on b.student_id = c.student_id
+where  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 c.* from 
+	(select * from score where course_id = 1) a join 
+	(select * from score where course_id = 2) b on a.student_id = b.student_id join 
+	student c on b.student_id = c.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 
+(select * from score where course_id = 1) a left join 
+(select * from score where course_id = 2) b on a.student_id = b.student_id where b.course_id is null;
+-- 4. 查询不存在" 1 "课程但存在" 2 "课程的情况
+select * from 
+(select * from score where course_id = 1) a right join 
+(select * from score where course_id = 2) b on a.student_id = b.student_id where a.course_id is null;
+-- 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 * from 
+(select  distinct a.student_id,student_name,avg(score) over (partition by student_id) 平均成绩 from student a join score b on a.student_id= b.student_id) a
+where 平均成绩 >= 60;  
+-- 6. 查询在成绩表存在成绩的学生信息
+select a.* from student a,
+(select distinct student_id from score) b 
+where a.student_id = b.student_id;
+-- 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 a.student_id ,
+ a.student_name ,
+ count(course_id) over (partition by student_id) 选课总数,
+ sum(score) over (partition by student_id) 总成绩
+ from student a left join score b
+	on a.student_id = b.student_id;
+-- 8. 查询「李」姓老师的数量
+select count(teacher_name) 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 a.* from student a , score b , course c,teacher d 
+ where a.student_id = b.student_id 
+ and b.course_id = c.course_id 
+ and c.teacher_id = d.teacher_id 
+ and d.teacher_name = '李白';
+-- 10. 查询没有学全所有课程的同学的信息
+SELECT t.* from score s,student t where s.student_id=t.student_id  GROUP BY t.student_id HAVING count(s.course_id) > (SELECT count(course_id) from course);
+-- select * from student a left join score b on a.student_id = b.student_id right join course c on b.course_id = c.course_id;
+
+-- 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 distinct a.* from student a left join score b on a.student_id = b.student_id 
+ where  course_id in
+ (select course_id from score where 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
+
+
+-- 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 a 
+-- left join score b  on a.student_id = b.student_id 
+-- left join  course c on b.course_id = c.course_id 
+-- left join teacher d on c.teacher_id = d.teacher_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
+
+-- 15. 检索" 1 "课程分数小于 60，按分数降序排列的学生信息
+select b.* from
+(select * from score where course_id = 1 and score < 60 order by score desc) a left join student b on a.student_id= b.student_id;
+-- 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 * from 
+(select student_id 学生id,course_id 课程id,score 成绩,avg(score) over (partition by student_id) 平均成绩 from score) a
+order by 平均成绩 desc;
+-- 17. 查询各科成绩最高分、最低分和平均分
+select course_id 课程编号,max(score) 最高分,min(score) 最低分,avg(score) 平均分 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 * from  
+-- 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 student_id,course_name,score,row_number() over(partition by student_id order by score desc) 排名 
+	from score sco left join course c on sco.course_id = c.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 student_id,course_name,score,dense_rank() over(partition by student_id order by score desc) 排名 
+	from score sco left join course c on sco.course_id = c.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 *,row_number() over(order by a.总分 desc) from 
+	(select stu.student_id,student_name, sum(score) 总分 
+	from student stu left join score sco on stu.student_id = sco.student_id 
+	group by stu.student_id order by 总分 desc) 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 *,rank() over(order by a.总分 desc) from 
+	(select stu.student_id,student_name, sum(score) 总分 
+	from student stu left join score sco on stu.student_id = sco.student_id 
+	group by stu.student_id order by 总分 desc) 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 sco.course_id,course_name,
+	count(score<60 or null) `[60-0]人数`,
+	count(score>=60 and score<=70 or null) `[70-60]人数`,
+	count(score >70 and score <=85 or null) `[85-70]人数`,
+	count(score>85 or null) `[100-85]人数`,
+	concat(round(count(score<60 or null)/count(sco.course_id)*100,2),'%') `[0-60]所占百分比`,
+	concat(round(count(score>=60 and score<=70 or null)/count(sco.course_id)*100,2),'%')`[60-70]所占百分比`,
+	concat(round(count(score >70 and score <=85 or null)/count(sco.course_id)*100,2),'%') `[70-85]所占百分比`,
+	concat(round(count(score>85 or null)/count(sco.course_id)*100,2),'%') `[100-85]所占百分比`
+	from score sco left join course c on sco.course_id = c.course_id 
+	group by sco.course_id ;
+-- 24. 查询各科成绩前三名的记录
+select * from 
+	(select student_id,course_name,score,row_number() over(partition by sco.course_id order by score desc) 排名
+	from score sco left join course c on sco.course_id = c.course_id) a where a.排名 < 4
+-- 25. 查询每门课程被选修的学生数
+select course_id,count(course_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 * from student s where (select count(*) from score sc where s.student_id=sc.student_id)<>(select count(*) from course);
+-- 27. 查询男生、女生人数
+SELECT 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 count(*) from student GROUP BY student_name,gender HAVING count(*)>1;
+-- 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 course_name,avg(score) from score s LEFT JOIN course c on s.course_id=c.course_id GROUP BY c.course_id ORDER BY avg(score) desc,c.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 s.student_id,s.student_name,avg(score) from student s LEFT JOIN score c on s.student_id=c.student_id GROUP BY s.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 DISTINCT s.student_name,score from student s LEFT JOIN score c on s.student_id=c.student_id LEFT JOIN course o on c.course_id=o.course_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
+	s.student_id,
+	s.student_name,
+	c.course_name,
+	sc.score 
+FROM
+	score sc
+	RIGHT JOIN student s ON sc.student_id = s.student_id
+	LEFT JOIN course c ON sc.course_id = c.course_id 
+GROUP BY
+	s.student_id,
+	s.student_name,
+	c.course_name,
+	sc.score;
+-- 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 s.student_name,c.course_name,sc.score from student s,score sc,course c where s.student_id=sc.student_id and sc.course_id=c.course_id and sc.score>70;
+-- 36. 查询不及格的课程
+select distinct c.course_name from score s,course c  where s.course_id=c.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 s.student_id,s.student_name,sc.score from student s,score sc where s.student_id=sc.student_id and sc.course_id=1 and sc.score in (select score from score where score>=80);
+-- 38. 求每门课程的学生人数
+select course_id,count(course_id) from score group by course_id;
+-- 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
+	s.* ,sc.score
+FROM
+	student s,
+	score sc,
+	course c,
+	teacher t 
+WHERE
+	s.student_id = sc.student_id 
+	AND sc.course_id = c.course_id 
+	AND c.teacher_id = t.teacher_id 
+	AND sc.score IN (
+	SELECT
+		max( score ) 
+	FROM
+		score sc,
+		course c,
+		teacher t 
+	WHERE
+		sc.course_id = c.course_id 
+		AND c.teacher_id = t.teacher_id 
+		AND t.teacher_name = '高斯' 
+	);
+-- 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
+	distinct s.* ,sc.score
+FROM
+	student s,
+	score sc,
+	course c,
+	teacher t 
+WHERE
+	s.student_id = sc.student_id 
+	AND sc.course_id = c.course_id 
+	AND c.teacher_id = t.teacher_id 
+	AND sc.score IN (
+	SELECT
+		max( score ) 
+	FROM
+		score sc,
+		course c,
+		teacher t 
+	WHERE
+		sc.course_id = c.course_id 
+		AND c.teacher_id = t.teacher_id 
+		AND t.teacher_name = '高斯' 
+	);
+-- 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 stu.student_id,course_id,score 
+	from student stu left join score sco on stu.student_id = sco.student_id 
+	where stu.student_id in
+	(select distinct s1.student_id from score s1,score s2 where s1.course_id != s2.course_id and s1.score = s2.score);
+-- 42. 查询每门功成绩最好的前两名
+select * from 
+	(select student_id,course_name,score,row_number() over(partition by sco.course_id order by score desc) 排名
+	from score sco left join course c on sco.course_id = c.course_id ) a
+	where a.排名 <=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(course_id) from score GROUP BY course_id having count(course_id)>=5;
+-- 44. 检索至少选修两门课程的学生学号
+select  s.student_id,count(sc.course_id) from student s,score sc where s.student_id=sc.student_id group by s.student_id having count(sc.course_id)>2;
+-- 45. 查询选修了全部课程的学生信息
+select s.*, count(sc.student_id) from student s,score sc where s.student_id=sc.student_id group by sc.student_id having count(sc.student_id)=3;
+-- 46. 查询各学生的年龄，只按年份来算
+select year(now())-year(birthday) 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,timestampdiff(year,birthday,now()) 年龄 from student;
+-- 48. 查询本周过生日的学生
+select * from student where weekday(student.birthday)=weekday(now());
+-- 49. 查询下周过生日的学生
+select * from student where weekday(student.birthday)=weekday(now())+1;
+-- 50. 查询本月过生日的学生
+select * from student where month(student.birthday)=month(now());
+-- 51. 查询下月过生日的学生
+select * from student where month(birthday)=month(now())+1;
+```
+````
+