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一张图搞定七种 JOIN 关系

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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一张图搞定七种JOIN关系

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
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一张图搞定七种JOIN关系

一张图搞定七种JOIN关系

2. 分别填充数据 
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一张图搞定七种JOIN关系

一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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;
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一张图搞定七种JOIN关系

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