在 mysql 查詢語句中，JOIN 扮演的角色很重要，所以掌握其用法很重要。很多同學可能只是會用幾種常用的，但要成為高階的工程師是需要掌握透徹，360度全無死角。
圖片精華版
文字解釋版
1. 需要準備好兩個table：subject（學科表）和 student_score（學生成績表）
通過學生成績表的subject_id欄位（學科ID）和學科表的id欄位（主鍵ID）進行關聯
2. 分別填充資料 
3. inner join
語句：select score.student_name,score.score,subject.name,subject.teacher from student_score as score inner join subject on score.subject_id = subject.id;
4. left join （共有+右表不匹配補NULL）
語句：select score.student_name,score.score,subject.name,subject.teacher from student_score as score left join subject on score.subject_id = subject.id;
5. left join （左表獨有）
語句：select score.student_name,score.score,subject.name,subject.teacher from student_score as score left join subject on score.subject_id = subject.id where subject.id is null;
6. right join （共有+左表不匹配補NULL）
語句：select score.student_name,score.score,subject.name,subject.teacher from student_score as score right join subject on score.subject_id = subject.id;
7. right join （右表獨有）
語句：select score.student_name,score.score,subject.name,subject.teacher from student_score as score right join subject on score.subject_id = subject.id where score.id is null;
8. union （左右表合併並去重）
語句：
select score.student_name,score.score,subject.name,subject.teacher from student_score as score left join subject on score.subject_id = subject.id
union
select score.student_name,score.score,subject.name,subject.teacher from student_score as score right join subject on score.subject_id = subject.id;
9. union （左右表獨有）
語句：
select score.student_name,score.score,subject.name,subject.teacher from student_score as score left join subject on score.subject_id = subject.id where subject.id is null
union
select score.student_name,score.score,subject.name,subject.teacher from student_score as score right join subject on score.subject_id = subject.id where score.id is null;