sql内外连接

笛卡尔积 3 × 2

select * from t_user, t_address;
或
select * from t_user inner join t_address;

左外链接

select * from t_user u 
left outer join t_address a on u.id = a.user_id;

右外连接

select * from t_user u 
right outer join t_address a on u.id = a.user_id;

https://blog.csdn.net/javacoder_juejue/article/details/83037521