在 mysql 查询语句中，JOIN 扮演的角色很重要，所以掌握其用法很重要。很多同学可能只是会用几种常用的，但要成为高级的工程师是需要掌握透彻，360度全无死角。

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文字解释版

1. 需要准备好两个table：subject（学科表）和 student_score（学生成绩表）
通过学生成绩表的subject_id字段（学科ID）和学科表的id字段（主键ID）进行关联

2. 分别填充数据 

3. inner join
语句：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+右表不匹配补NULL）
语句：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 （左表独有）
语句：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+左表不匹配补NULL）
语句：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 （右表独有）
语句：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 （左右表合并并去重）
语句：
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 （左右表独有）
语句：
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;