数据蛙测试题 - 20200715

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。结果如下

代码：

select st.student_id,st.student_name,su.subject_name, count(su.subject_name) as attended_exams

from

students st cross join subjects su

left join Examinations as e

on e.student_id = st.student_id

group by st.student_id,su.subject_name

order by st.student_id , su.subject_name

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学到新【知识点】cross join 返回两表的笛卡尔积，具体如下