为什么我的SELECT语句显示重复记录？

Question

我在克服这个错误方面遇到了一些麻烦。当我完整地执行SELECT语句时，会显示一条重复的消息。

的代码是：

SELECT students.first_name, last_name ,entries.subject_id, subject_name, level_of_entry 
FROM students, subjects , entries 
WHERE subjects.subject_id=entries.subject_id 
AND students.student_id=entries.student_id 
AND exam_board='ocr'; 

This is what happens after that code is processed

These are the tables which the data is coming from

有没有办法停止复制或删除重复的行吗？

复制的行是第6和第7行;一个与Zeynnata Rein。

感谢您的帮助！

PS：对不起，物理学

Answer 1

拼写错误，你必须通过student_id数据

Answer 2

使用组，因为你不加入它返回所有匹配组合的表。尽量明确连接：

SELECT st.first_name, st.last_name, e.subject_id, su.subject_name, su.level_of_entry 
FROM students st 
LEFT JOIN entries e 
ON st.student_id = e.student_id 
LEFT JOIN subjects su 
ON e.subject_id = su.subject_id 
WHERE su.exam_board = 'ocr';