几种常见 join 的用法
创建表
插入数据
INNER JOIN
select * from join_user u inner join join_order o on u.id = o.user_id
结果如下：
结论：在表中存在至少一个匹配时，INNER JOIN 关键字返回行。
LEFT JOIN
select * from join_user u left join join_order o on u.id = o.user_id
结果如下：
结论：left join 会返回左表所有的行，即使右表中没有匹配的行
select * from join_user u left join join_order o on u.id = o.user_id WHERE o.user_id is null
结果如下：
结论：返回左表特有的行
RIGHT JOIN
select * from join_user u right join join_order o on u.id = o.user_id
结果如下：
结论：right join 会返回右表所有的行，即使左表中没有匹配的行
select * from join_user u right join join_order o on u.id = o.user_id WHERE u.id is null
结果如下：
结论：会返回右表中独有的行
full join
MySQL 不支持 full join，通过left join 、right join、union 组合实现
select * from join_user u right join join_order o on u.id = o.user_id UNION select * from join_user u left join join_order o on u.id = o.user_id
结果如下：
两表中都没有出现的数据
select * from join_user u right join join_order o on u.id = o.user_id WHERE u.id is null UNION select * from join_user u left join join_order o on u.id = o.user_id WHERE o.user_id is null
结果如下：
