LeetCode-1280. 学生们参加各科测试的次数（简单）

题目来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/students-and-examinations
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本人思路：
– 1、观察Result table，前3列是Students table和Subjects table的笛卡尔积连接，构成temp表
– 2、将Examinations table按student_id,subject_name分组，计算每个学生参加每一门科目测试的次数，构成temp1表
– 3、将temp表与temp1表左连接

select temp.student_id,temp.student_name,temp.subject_name,ifnull(temp1.counts,0) as attended_exams 
from(select s1.student_id,s1.student_name,s2.subject_name 
    from Students as s1 ,Subjects as s2
    )temp left join(
        select student_id,subject_name,count(subject_name) as counts
        from Examinations  
        group by student_id,subject_name
        )temp1
on temp.student_id=temp1.student_id and temp.subject_name=temp1.subject_name
order by student_id , subject_name