题目
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。
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。
考试表: Examinations
+--------------+---------+
| Column Name | Type |
+--------------+---------+
| student_id | int |
| subject_name | varchar |
+--------------+---------+
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 |
+------------+--------------+
Subjects table:
+--------------+
| subject_name |
+--------------+
| Math |
| Physics |
| 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 |
+------------+--------------+
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。
解答
与Leetcode1127类似
想要包含所有的结果 利用笛卡尔乘积即可
果不带WHERE条件子句，它将会返回被连接的两个表的笛卡尔积，返回结果的行数等于两个表行数的乘积；
SELECT *
FROM Students as st, Subjects as su
然后与Examinations表左连接
SELECT *
FROM Students as st, Subjects as su
LEFT JOIN Examinations as e ON st.student_id = e.student_id AND su.subject_name = e.subject_name
按id和科目分组 统计数量即可
SELECT st.student_id, st.student_name, su.subject_name,
count(e.student_id) as attended_exams
FROM Students as st, Subjects as su
LEFT JOIN Examinations as e ON st.student_id = e.student_id AND su.subject_name = e.subject_name
group by st.student_id, su.subject_name
order by st.student_id, su.subject_name
这里不能用COUNT(*)，要用COUNT(e.subject_name)，因为e.subject_name中才有NULL