left joinを使用してNULL値付きデータテーブルを接続するケース

insert into t_user values(1,'use1','123','user1',1,0,'[email protected]',0,now());
insert into t_user values(2,'use2','123','user2',1,0,'[email protected]',0,now());

insert into t_message values(1,'msg11','content11',now(),1);
insert into t_message values(2,'msg12','content12',now(),1);
insert into t_message values(3,'msg21','content21',now(),1);
insert into t_message values(4,'msg22','content22',now(),1);

insert into t_attachment values (1,'attach1','attach1','zip',now(),1);
insert into t_attachment values (2,'attach2','attach1','zip',now(),1);

select t_message.m_id,a_id from t_message,t_attachment where t_message.m_id = t_attachment.m_id;
select t_message.u_id,m_id from t_user,t_message where t_message.u_id = t_user.u_id;
#left join        ，         ，        NULL
select * from ((select t_message.u_id,m_id from t_user,t_message where t_message.u_id = t_user.u_id) as a left join (select t_message.m_id,a_id from t_message,t_attachment where t_message.m_id = t_attachment.m_id) as b on a.m_id = b.m_id);

結果セット:u_id | m_id | m_id | a_id ——+——+——+—— 1 | 1 | 1 | 1 1 | 1 | 1 | 2 1 | 2 | |
1 | 3 | |
1 | 4 | |
(5 rows)