一张图搞定七种 JOIN 关系

编程并不是一个机械性的工作，而是需要有思考，有创新的工作，语法是固定的，但解决问题的思路则是依靠人的思维，这就需要我们坚持学习和更新自己的知识。今天golang学习网就整理分享《一张图搞定七种 JOIN 关系》，文章讲解的知识点主要包括MySQL，如果你对数据库方面的知识点感兴趣，就不要错过golang学习网，在这可以对大家的知识积累有所帮助，助力开发能力的提升。

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;

今天带大家了解了MySQL的相关知识，希望对你有所帮助；关于数据库的技术知识我们会一点点深入介绍，欢迎大家关注golang学习网公众号，一起学习编程~