sql语句-关于自身表连接之join与left join
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1、创建表
drop table if exists t_user;create table t_user(id int(11) not null auto_increment,user_id int(11),user_name varchar(100),primary key(id))engine=Innodb default charset=utf8;
2、插入测试数据
insert into t_user values(1,1,'admin');insert into t_user values(2,2,'superadmin');
3、表自身关联-join
select * from t_user t1 join (select * from t_user) t2 on t1.id=1 and t2.id=2
4、表自身关联-left join
select * from t_user t1 left join (select * from t_user) t2 on t1.id=1 and t2.id=2
总结:
1)表的自关联是通过把一个表命名不同的名称,然后找到想要关联的关系进行关联;
2)左连接left join查询出来的结果不仅仅是on 后面条件关联的数据,还包括条件中没有关联到的但是左边的select查询到的结果,没有关联到的数据部分用NULL填充。
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