练习题笔记0715

MySQL基础

1、如果暂停或开启MySQL服务？

net stop mysql80(版本号)
net start mysql80(版本号)

2、如何使用cmd命令登录MySQL窗口

mysql -u 用户名 -p 密码

3、登录时用到-u \ -p等参数，如何查看所有参数意思？

mysql --help

MySQL练习题

习题

学生表：students

Column Name	Type
student_id	int
student_name	varchar

创建表

create table if not exists Students (student_id int primary key,student_name varchar(20));

插入数据

Truncate table students;
insert into students (student_id, student_name) values ('1', 'Alice');
insert into students (student_id, student_name) values ('2', 'Bob');
insert into students (student_id, student_name) values ('13', 'John');
insert into students (student_id, student_name) values ('6', 'Alex');

科目表：subjects

Column Name	Type
subject_name	varchar

创建表

create table if not exists subjects (subject_name varchar(20)  primary key);

插入数据

Truncate table Subjects;
insert into subjects (subject_name) values ('Math');
insert into subjects (subject_name) values ('Physics');
insert into subjects (subject_name) values ('Programming');

考试表：examinations

Column Name	Type
student_id	int
subject_name	varchar

创建表

create table if not exists examinations (student_id int, subject_name varchar(20));

插入数据

Truncate table Examinations;
insert into examinations (student_id, subject_name) values ('1', 'Math');
insert into examinations (student_id, subject_name) values ('1', 'Physics');
insert into examinations (student_id, subject_name) values ('1', 'Programming');
insert into examinations (student_id, subject_name) values ('2', 'Programming');
insert into examinations (student_id, subject_name) values ('1', 'Physics');
insert into examinations (student_id, subject_name) values ('1', 'Math');
insert into examinations (student_id, subject_name) values ('13', 'Math');
insert into examinations (student_id, subject_name) values ('13', 'Programming');
insert into examinations (student_id, subject_name) values ('13', 'Physics');
insert into examinations (student_id, subject_name) values ('2', 'Math');
insert into examinations (student_id, subject_name) values ('1', 'Math');

要求写一段SQL，查询出每个学生参加每一门科目测试的次数，结果按student_id和subject_name排序。

select
a.student_id,
a.student_name,
b.subject_name,
count(c.subject_name) as attend_exams
from students as a
join subjects as b
left join examinations as c
on a.student_id=c.student_id and b.subject_name=c.subject_name
group by a.student_id,a.student_name,b.subject_name
order by a.student_id,b.subject_name

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