Mysql-外连接

-- 左连接：主表 left join 从表 on 连接条件；
-- left join
-- 查询每位学生对应的班级信息
select * from student inner join classess on student.cls_id=classess.id;
select * from student left join classess on student.cls_id=classess.id;
-- select * from student right join classess on student.cls_id=classess.id; 

-- 查询没有对应班级信息的学生
-- select ... from xxx as s left join xxx as c on ... where ...
-- select ... from xxx as s left join xxx as c on ... having ...
select * from student left join classess on student.cls_id=classess.id where classess.name is null;

-- 右连接： 从表 right join 主表 on 连接条件；
-- right join on
-- 将数据表名字互换位置，用left join完成

 

表取自：

Mysql-几张供于学习的表 - 夜黎i - 博客园 (cnblogs.com)