Question

我想用id中的users_tb更改数字2。我该如何补充，users_tb与其他表没有关系，它只是每次在某些活动上更新的一个列ID

SELECT messages.message,messages.sent_date, receiver_user.username, sender_user.username  FROM messages
        INNER JOIN users AS sender_user ON messages.msg_fk_user_id = sender_user.id
        INNER JOIN users AS receiver_user ON messages.receiver_id = receiver_user.id
        WHERE msg_fk_user_id = 1 AND receiver_id = 2 OR msg_fk_user_id = 2 AND receiver_id = 1
        ORDER BY `sent_date`;

Answer 1

尝试使用CROSS JOIN

SELECT messages.message,
       messages.sent_date, 
       receiver_user.username, 
       sender_user.username  
FROM messages
INNER JOIN users AS sender_user ON messages.msg_fk_user_id = sender_user.id
INNER JOIN users AS receiver_user ON messages.receiver_id = receiver_user.id
CROSS JOIN users_tb
WHERE msg_fk_user_id = 1 
AND receiver_id = users_tb.id
OR msg_fk_user_id = users_tb.id 
AND receiver_id = 1
ORDER BY `sent_date`

交叉联接返回行的笛卡尔积从两个表。与INNER JOIN或LEFT JOIN不同，交叉联接在连接的表之间不建立关系。 ...在一般，如果第一个表有n行，第二个表有m行行，交叉连接将产生n x m行。

我不知道数据库的结构，但是如果您想使用上面的查询，我认为您需要在users_tb表上插入一些过滤器。或者让我知道您是如何获得 2号的，我将通过改善查询方式来帮助您，例如：

...
CROSS JOIN (SELECT * FROM users_tb ORDER BY id DESC LIMIT 1) as users_tb
...

如何更改从其他数据库表中读取数字的条件？

1 个答案: