七、Mysql练习题目
https://zhuanlan.zhihu.com/p/38354000
1. 创建表
创建学生表
mysql> create table if not exists student(
-> student_id varchar(255) not null,
-> student_name varchar(255) not null,
-> birthday date not null,
-> gender varchar(255) not null,
-> primary key(student_id)
-> )default charset utf8;
创建成绩表
mysql> create table score(
-> student_id varchar(255) not null,
-> course_id varchar(255) not null,
-> score float(3) not null,
-> primary key(student_id,course_id)
-> )default charset utf8;
创建课程表
mysql> create table course(
-> course_id varchar(255) not null,
-> course_name varchar(255) not null,
-> teacher_id varchar(255) not null,
-> primary key(course_id)
-> )default charset utf8;
创建教师表
mysql> create table teacher(
-> teacher_id varchar(255) not null,
-> teacher_name varchar(255) null,
-> primary key(teacher_id)
-> ) default charset utf8;
2. 插入数据
学生表插入数据
insert into student
values
('0001','猴子','1989-01-01','男'),
('0002','猴子','1990-12-21','女'),
('0003','马云','1991-12-21','男'),
('0004','王思聪','1990-05-20','男');
成绩表插入数据
insert into score
values
('0001','0001',80),
('0001','0002',90),
('0001','0003',99),
('0002','0002',60),
('0002','0003',80),
('0003','0001',80),
('0003','0002',80),
('0003','0003',80);
课程表插入数据
insert into course
values
('0001','语文','0002'),
('0002','数学','0001'),
('0003','英语','0003');
教师表插入数据
insert into teacher
values
('0001','孟扎扎'),
('0002','马化腾'),
('0003',null),
('0004','');
3. 题目
1.查询姓“猴”的学生名单
select * from student where student_name like "猴%";
2.查询名字中最后一个字是猴的学生名单
select * from student where student_name like "%猴";
3.查询名字中带猴的学生名单
select * from student where student_name like "%猴%";
4.查询名字中第二个字是猴的学生名单
select * from student where student_name like "_猴%";
5.查询姓“孟”老师的个数
关键字：个数-count
select count(*) from teacher where teacher_name like "孟%";
6.查询课程编号为“0002”的总成绩
关键字：总成绩-sum
select sum(score) from score where course_id='0002';
7.查询选了课程的学生人数*
关键字：人数-count
select 学号，成绩表里学号有重复值需要去掉
select count(distinct student_id) as 学生人数 from score;
8.查询各科成绩最高和最低的分
关键字：各科-分组；最高分-max ；最低分-min
select course_id,min(score) as 最低分,max(score) as 最高分 from score group by course_id;
9.查询每门课程被选修的学生数
关键字：每门-分组；学生数-count
select course_id as 课程,count(student_id) as 学生个数 from score group by course_id;
10.查询男生、女生人数
关键字：男生、女生（相当于每，因为性别只有男、女）-分组；人数-count
select gender,count(*) from student group by gender;
11.查询平均成绩大于60分学生的学号和平均成绩
关键字：平均成绩大于-having avg()>
select student_id,avg(score) from score group by student_id having avg(score)>60;
12.查询至少选修两门课程的学生学号
关键字：至少-count
select student_id from score group by student_id having count(course_id)>=2;
13.查询同名同姓学生名单并统计同名人数*
关键字：人数-count
select student_name,count(student_id) from student group by student_name having count(student_id) >=2;
14.查询不及格的课程并按课程号从大到小排列
select * from score where score<60 order by course_id desc;
15.查询每门课程的平均成绩，结果按平均成绩升序排序，平均成绩相同时，按课程号降序排列
关键字：每门-group by；平均成绩-avg
select course_id,avg(score) as 平均成绩 from score group by course_id order by 平均成绩,course_id desc;
16.检索课程编号为“0004”且分数小于60的学生学号，结果按按分数降序排列
select student_id student from score where course_id='0004' and score<60 order by score desc;
17.统计每门课程的学生选修人数(超过2人的课程才统计)
关键字：每门-group by；人数-count
select course_id,count(student_id) from score group by course_id having count(student_id)>2;
18.查询两门以上不及格课程的同学的学号及其平均成绩*
19.查询每个学生的总成绩并进行排名
关键字：每个-group by；总成绩-sum
select student_id,sum(score) as 总成绩 from score group by student_id order by 总成绩;
20.查询平均成绩大于60分的学生的学号和平均成绩
关键字：平均成绩大于-having avg()>60
select student_id,avg(score) as 平均成绩 from score group by student_id having avg(score)>60;
21.查询所有课程成绩小于60分学生的学号、姓名
关键字：所有成绩小于60-where score < 60
因为姓名在student表中，成绩在score表中，所以要用到子查询。
注意：如果(值1,值2,……)存在重复值时，in (值1,值2,……) 会从(值1,值2,……)中的重复值中选择一个。即in会过滤掉重复数据
select student_id,student_name from student where student_id in (select student_id from score where score<60);
22.查询没有学全所有课的学生的学号、姓名*
select student_id,student_name from student where student_id in (select student_id from score group by student_id having count(course_id) < (select count(*) from course));
23.查询出只选修了两门课程的全部学生的学号和姓名
select student_id,student_name from student where student_id in (select student_id from score group by student_id having count(course_id)=2);
24.查询各科成绩前两名的记录*
关键字：各科-group by
https://mp.weixin.qq.com/s/MuxjlFV0gi1GydOrYfiSeQ
25.查找1990年出生的学生名单*
select * from student where year(birthday) = 1990;
或
select * from student where birthday like '1990-%';
26.查询各学生的年龄（精确到年）*
select student_id, year(current_time)-year(birthday)+1 from student;
27.查询各学生的年龄（精确到月份）*
select student_id,timestampdiff(month ,birthday ,now())/12 from student;
28.查询本月过生日的学生*
select * from student where month(birthday)=month(current_date);
29.查询所有学生的学号、姓名、选课数、总成绩*
要显示的列：学号，姓名，选课数，总成绩，分布在两个表中，所以应该用连表查询，将两个表连接起来
关键字：所有学生-左连接或右连接：左连接的话join左边的表应该为student，右连接的话join右边应该为student表。
注意：分组时，如果要用外连接的条件分组，则选择的那个表中的列中的数据应该是唯一的，及s.student_id
select s.student_id as 学号,s.student_name as 姓名,count(c.course_id) as 选课数,sum(c.score) as 总成绩 from student as s left join score as c on s.student_id=c.student_id group by s.student_id;
30.查询平均成绩大于85的所有学生的学号、姓名和平均成绩*
select t1.student_id as '学号',t1.student_name as '姓名',avg(score) as '平均成绩' from student as t1 left join score as t2 on t1.student_id = t2.student_id group by t1.student_id having avg(t2.score)>85;
31.查询学生的选课情况：学号，姓名，课程号，课程名称*
select t1.student_id,t1.student_name,t3.course_id,t3.course_name from student as t1 left join score as t2 on t1.student_id=t2.student_id left join course as t3 on t2.course_id=t3.course_id;
32.查询出每门课程的及格人数和不及格人数*
关键字：及格和不及格-分类：case语句
select course_id, sum(case when score>=60 then 1 else 0 end) as 及格人数, sum(case when score<60 then 1 else 0 end) as 不及格人数 from score group by course_id;
33.使用分段[100-85],[85-70],[70-60],[<60]来统计各科成绩，分别统计：各分数段人数，课程号和课程名称
select a.课程号,b.课程名称,
sum(case when 成绩 between 85 and 100
then 1 else 0 end) as '[100-85]',
sum(case when 成绩 >=70 and 成绩<85
then 1 else 0 end) as '[85-70]',
sum(case when 成绩>=60 and 成绩<70
then 1 else 0 end) as '[70-60]',
sum(case when 成绩<60 then 1 else 0 end) as '[<60]'
from score as a right join course as b
on a.课程号=b.课程号
group by a.课程号,b.课程名称;
34.查询课程编号为0003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='0003' and score>80);
35.检索"0001"课程分数小于60，按分数降序排列的学生信息
select t1.*,t2.score from student as t1 left join score as t2 on t1.student_id=t2.student_id where t2.course_id = 0001 and t2.score>60 order by t2.score desc;
36.查询不同老师所教不同课程平均分从高到低显示**
注意：select只能是聚合函数或group by后面的字段，否则会报SELECT list is not in GROUP BY clause and contains nonaggregated column的错误。原因是sql_mode模式的限制。可以修改这个模式，修改方法：https://blog.csdn.net/weixin_42085125/article/details/115335503
因为我们要查询t2.course_id，所以要在group by后面加上t2.course_id
select t1.teacher_name,t2.course_id,avg(t3.score) from teacher as t1 inner join course as t2 on t1.teacher_id=t2.teacher_id inner join score as t3 on t2.course_id=t3.course_id group by t1.teacher_id,t2.course_id order by avg(score) desc;
37.查询课程名称为"数学"，且分数低于60的学生姓名和分数
行列如何互换
sql面试题：行列如何互换？
