图片精华版
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;