left join,right join,join的区别

left join：返回包括左表中的所有记录和右表中联结字段相等的记录

right join ：返回包括右表中的所有记录和左表中联结字段相等的记录

join(inner join) : 只返回两个表中联结字段相等的行

实例：

1.student表

2.score表

3.left join 结果

select a.name,b.subject_name,b.score from student a left join score b on a.id = b.student_id;

4.right join 结果

select a.name,b.subject_name,b.score from student a right join score b on a.id = b.student_id;

5.join 结果

select a.name,b.subject_name,b.score from student a join score b on a.id = b.student_id;

等价于：

select a.name,b.subject_name,b.score from student a,score b where a.id = b.student_id;

欢迎纠错。