mysql基础
1、如何暂停或开启mysql服务？
net stop mysql / net start mysql
2、如何用cmd命令登录mysql窗口
先进入到安装的MySQL的bin目录下
mysql-h主机地址-u用户名-p用户密
3、登录时mysql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
主键为 student_id（学生ID），该表内的每一行都记录有学校一名学生的信息。
create table if not exists Students (student_id int , student_name varchar(20));
科目表: Subjects
Column Name Type
subject_name varchar
主键为 subject_name（科目名称），每一行记录学校的一门科目名称。
Create table If Not Exists Subjects (subject_name varchar(20));
考试表: Examinations
Column Name Type
student_id int
subject_name varchar
Create table If Not Exists Examinations (student_id int, subject_name varchar(20));
这张表压根没有主键，可能会有重复行。
学生表里的一个学生修读科目表里的每一门科目，而这张考试表的每一行记录就表示学生表里的某个学生参加了一次科目表里某门科目的测试。
要求写一段 SQL 语句，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。
查询结构格式如下所示：
Students table:
student_id student_name
1 Alice
2 Bob
13 John
6 Alex
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');
Subjects table:
subject_name
Math
Physics
Programming
Truncate table Subjects;
insert into Subjects (subject_name) values ('Math');
insert into Subjects (subject_name) values ('Physics');
insert into Subjects (subject_name) values ('Programming');
Examinations table:
student_id subject_name
1 Math
1 Physics
1 Programming
2 Programming
1 Physics
1 Math
13 Math
13 Programming
13 Physics
2 Math
1 Math
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');
Result table:
student_id student_name subject_name attended_exams
1 Alice Math 3
1 Alice Physics 2
1 Alice Programming 1
2 Bob Math 1
2 Bob Physics 0
2 Bob Programming 1
6 Alex Math 0
6 Alex Physics 0
6 Alex Programming 0
13 John Math 1
13 John Physics 1
13 John Programming 1
结果表需包含所有学生和所有科目（即便测试次数为0）：
Alice 参加了 3 次数学测试, 2 次物理测试，以及 1 次编程测试；
Bob 参加了 1 次数学测试, 1 次编程测试，没有参加物理测试；
Alex 啥测试都没参加；
John 参加了数学、物理、编程测试各 1 次。
