学生们参加各科测试的次数

题目描述：

解题思路：

它要所有学生的所有科目的测试数目，所以，students和subjects笛卡尔积，然后examinations以它左连接

select st.student_id,st.student_name,st.student_name,count(ex.student_id) attended_exams
from students st
    join subjects su
    left join examinations ex
    on st.student_id = ex.student_id and su.subject_name = ex.subject_name
group by st.student_id,su.subject_name;

表结构：

create table students(
    student_id int,
    student_name varchar(150),
    primary key (student_id)
);

create table subjects(
    subject_name varchar(150),
    primary key (subject_name)
);

create table Examinations(
    student_id int,
    subject_name varchar(150)
);

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

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');

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');