SQL多表查询
多表连接必须有连接条件,否则结果没有意义
隐式连接和显示内连接
select title,content,name,publish_time from user u,forum f #给表起⼀个别名,⽅便书写 where u.id = f.uid; 在where写链接条件 select title,content,name,publish_time from user u,forum f where u.id = f.uid and name='王琨';
显式内连接 交集
select username,name,title -> from bbs_user a inner join bbs_forum c on c.uid =a.uid -> inner join bbs_category b on c.cid = b.cid;
表的自身连接
select * from areainfo a,areainfo b where a.pid=b.code and a.name='⻘河县';
外连接
两张表关联查询根据以哪一张表为主可以分为左外连接和右外连接
select username,r.* from blog_user u left join blog_remark r on u.uid = r.uid
它们的区别是什么?
https://blog.csdn.net/xuanjiewu/article/details/50636465