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+## 50题综合练习
+
+``` mysql
+create database tt_db charset utf8mb4;
+use tt_db;
+
+
+# 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);
+
+
+select * from score;
+-- 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 student where student_id in(
+select student_id from
+(select * from score where course_id=1) a join
+(select * from score where course_id=2) b using(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 * from student where student_id in
+(select * from
+(select student_id from score where course_id=1) a join
+(select student_id from score where course_id=2) b using(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 a.*,b.course_id,b.score from
+(select * from score ) a left join 
+(select * from score where course_id in(2)) b on a.student_id=b.student_id;
+
+
+-- SELECT s.student_id, s.student_name  
+-- FROM student s  
+-- LEFT JOIN score sc1 ON s.student_id = sc1.student_id AND sc1.course_id = 1  
+-- LEFT JOIN score sc2 ON s.student_id = sc2.student_id AND sc2.course_id = 2  
+-- WHERE sc1.score IS NOT NULL AND sc2.score IS NULL;
+-- 4. 查询不存在" 1 "课程但存在" 2 "课程的情况
+select a.*,b.course_id,b.score from
+(select * from score ) a left join 
+(select * from score where course_id in(1)) b on a.student_id=b.student_id;
+-- 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 a.*,平均成绩 from
+(select * from student) a left join 
+(select student_id,avg(score) 平均成绩 from score group by student_id) b using(student_id) where 平均成绩>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 a.*,选课总数,总成绩 from
+(select * from student) a left join (select student_id,count(*) 选课总数,sum(score) 总成绩 from score group by student_id) b on a.student_id=b.student_id
+-- 8. 查询「李」姓老师的数量
+select count(*) from teacher where locate('李',teacher_name);
+-- 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=
+(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 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 student_id from score group by student_id having count(course_id)= 
+(select count(course_id) from score where 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 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')))
+-- 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 a.student_name,b.* from
+(select * from student) a join 
+(select student_id,avg(score) 平均成绩 from score where score<60 group by student_id having count(score)>=2) b using(student_id)
+-- 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
+(select * from student where student_id) a join
+(select student_id,score from score where course_id=1 and score<60) b using(student_id) 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 * from
+(select student_id,score 各科成绩,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 distinct course_id,max(score) over(partition by course_id) 最高成绩,min(score) over(partition by course_id) 最低成绩,avg(score) over(partition by course_id) 平均成绩 from score
+-- 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,
+max(s.score) 最高分,
+min(s.score) 最低分,
+avg(s.score) 平均分,
+count((case when s.score>=60 or null then '及格' end)) / count(*) 及格率,
+count((case when s.score>= 70 or null and s.score<80 then '中等' end)) / count(*) 中等率,
+count((case when s.score>=80 or null and s.score<90 then '优良' end)) / count(*) 优良率,
+count((case when s.score>=90 or null then '优秀' end)) / count(*) 优秀率,
+count(c.course_id) 选秀人数
+ from score s join course c on s.course_id=c.course_id group by c.course_id order by 选秀人数 desc ,c.course_id asc
+-- 19. 按各科成绩进行排序，并显示排名，Score 重复时保留名次空缺
+	select dense_rank() over(partition by course_id order by score) 排名,course_id,score 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) 排名,course_id,score 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  student_id,course_id,sum(score) over(partition by student_id) 学生总分,dense_rank() over(partition by course_id order by score desc) 排名 from score;
+
+-- 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 distinct student_id,a.总成绩,rank() over(order by 总成绩 desc) 排名 from
+(select student_id,sum(score) over(partition by student_id) 总成绩 from score) a join
+(select student_id,sum(score) over(partition by student_id) 总成绩 from score) b using(student_id,总成绩);
+-- 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 
+a.*,b.course_name
+from
+(
+select  
+course_id,
+count(case when score<60  or null then '超低低分' end) 超低低分,
+count(case when score<70  or null then '低分' end) 低分,
+count(case when score<85  or null then '良好分' end) 良好分,
+count(case when score<85  or null then '优秀分' end) 优秀分,
+count(case when score<60  or null then '超低低分' end) /count(*) 超低低分比,
+count(case when score<70  or null then '低分' end) /count(*) 低分比,
+count(case when score<85  or null then '良好分' end) /count(*) 良好分比,
+count(case when score<85  or null then '优秀分' end) /count(*) 优秀分比
+ from score group by course_id) a
+ join 
+(select distinct c.course_id,course_name from score s join course c on s.course_id=c.course_id) b using(course_id)
+-- 24. 查询各科成绩前三名的记录
+select * from
+(select student_id,course_id,score,row_number() over(partition by course_id order by score desc) 排名数 from score) a where a.排名数<4
+-- 25. 查询每门课程被选修的学生数
+
+select distinct course_id,count(*) over(partition by course_id) 学生人数 from score
+-- 26. 查询出只选修两门课程的学生学号和姓名
+select * from student where student_id in
+(select a.student_id from
+(select distinct student_id,count(*) over(partition by student_id) 选棵数 from score) a where a.选棵数=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 locate('风',student_name)
+-- 29. 查询同名同性学生名单，并统计同名人数
+select * from
+(select student_name,count(*) 同名人数 from student group by student_name) a where a.同名人数>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 * from
+(select distinct course_id,avg(score) over(partition by course_id) 平均成绩 from score) a order by a.平均成绩 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 * from
+(select * from student) s1 join
+(select * from
+(select distinct student_id,avg(score) over(partition by student_id) 平均成绩 from score) a where a.平均成绩>85) s2 using(student_id)
+-- 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 * from
+(select * from student) a join
+(select student_id,score from score where course_id=
+(select course_id from course where course_name='数学') and score<60) b using(student_id)
+-- 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 c.*,s.score,s.student_id from course c left join score s on c.course_id=s.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 a2.student_name,a3.course_name,a1.score from
+(select * from score where score>70) a1
+join
+student a2 join course a3 on a1.student_id=a2.student_id and a1.course_id=a3.course_id
+-- 36. 查询不及格的课程
+select course_name from course where course_id in
+(select distinct course_id from score 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 student_id,student_name from student where student_id in
+(select student_id from score where course_id=1 and score>=80)
+
+-- 38. 求每门课程的学生人数
+select distinct course_id,count(*) over(partition by course_id) from score
+-- 39. 成绩不重复，查询选修「张三」改正[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 * from
+ student a natural join
+(select student_id,score from score where course_id=
+(select course_id from course where teacher_id=(select teacher_id from teacher where teacher_name='李白')) order by score limit 1) b
+-- 40. 成绩有重复的情况下，查询选修「张三」改正[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 * from
+ student a natural join
+(select student_id,score from score where course_id=
+(select course_id from course where teacher_id=(select teacher_id from teacher where teacher_name='李白')) order by score limit 1) b
+-- 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 * from score
+with 
+a as (select student_id,course_id,score  from score where course_id=1),
+b as (select student_id,course_id,score  from score where course_id=2),
+c as (select student_id,course_id,score  from score where course_id=3)
+select  a.student_id,a.course_id,a.score,b.course_id,b.score,c.course_id,c.score from a inner join b  using(student_id) inner join c using(student_id) 
+where a.score=b.score or a.score=b.score or a.score=c.score or c.score=b.score
+-- 42. 查询每门功成绩最好的前两名
+select * from
+(select student_id,course_id,score,row_number() over(partition by course_id order by score desc) 排名 from score) a where a.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 * from
+ (select distinct course_id,count(*) over(partition by course_id) 人数 from score) a where a.人数>5
+-- 44. 检索至少选修两门课程的学生学号
+select distinct student_id from
+(select student_id,count(*) over(partition by student_id) 选课数 from score) a where a.选课数>=2
+-- 45. 查询选修了全部课程的学生信息
+select distinct student_id from
+(select student_id,count(*) over(partition by student_id) 选课数 from score) a where a.选课数=(select count(*) from course)
+-- 46. 查询各学生的年龄，只按年份来算
+select student_name,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()) from student
+-- 48. 查询本周过生日的学生
+select * from student where weekofyear(concat_WS('-',year(curdate()),month(birthday),day(birthday))) = WEEKofyear(now())
+-- 49. 查询下周过生日的学生
+select * from student where weekofyear(concat_WS('-',year(curdate()),month(birthday),day(birthday))) = (WEEKofyear(now())+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;
+```
+