笛卡尔积
1
| select * from stu,team;
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表关联
笛卡尔积表关联
通过在笛卡尔积后面加上限制条件来达到关联效果
1
| select * from stu,team where stu.uid=team.uid;
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内关联
1
2
3
| select * from stu inner join team on stu.uid=team.uid;
//可以不写inner
select * from stu join team on stu.uid=team.uid;
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三张表关联
1
| select * from stu join middle on stu.uid=middle.uid join team on middle.sid=team.sid;
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左关联与右关联
以一侧的表为主,即显示该表的所有数据,即使没有所关联的数据
1
2
| select * from stu left join team on stu.uid=team.uid;
select * from stu right join team on stu.uid=team.uid;
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自关联
自关联时必须起别名
1
| select * from stu as s1 join stu as s2 on s1.uid=s2.uid where s1.name='liming';
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