一張圖搞定七種 JOIN 關係

在 mysql 查詢語句中，JOIN 扮演的角色很重要，因此掌握其用法很重要。不少同窗可能只是會用幾種經常使用的，但要成爲高級的工程師是須要掌握透徹，360度全無死角。

圖片精華版

文字解釋版

1. 須要準備好兩個table：subject（學科表）和 student_score（學生成績表）
經過學生成績表的subject_id字段（學科ID）和學科表的id字段（主鍵ID）進行關聯
複製代碼

2. 分別填充數據 
複製代碼

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;
複製代碼

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;
複製代碼

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;
複製代碼

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;
複製代碼

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;
複製代碼

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;
複製代碼

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;
複製代碼