MySQL JOIN错误：每个派生表都必须有自己的别名

我已经看过S.O的可能解决方案，我的查询仍然无法正常工作。MySQL JOIN错误：每个派生表都必须有自己的别名

这里的查询：

SELECT `messages`.`from_id`, 
    `messages`.`to_id`, 
    `messages`.`message` 
FROM `messages` 
JOIN 
(
    SELECT `users`.`profile_pic` 
    FROM `users` AS `from_pic` 
    WHERE `users`.`id` = `from_id` 
) 
    ON `users`.`id` = `messages`.`from_id` 
JOIN 
(
    SELECT `users`.`profile_pic` 
    FROM `users` AS `to_pic` 
    WHERE `users`.`id` = `to_id` 
) 
    ON `users`.`id` = `messages`.`to_id` 
WHERE `messages`.`from_id` = 23 
    AND `messages`.`to_id` = 24 
ORDER BY `date_sent` DESC;

表users

id | profile_pic | name

表messages

message_id | from_id | to_id | message

来源

2017-05-24 nTuply

当您使用子查询，则需要对他们提供一个别名：

SELECT 
    `messages`.`from_id`, 
    `messages`.`to_id`, 
    `messages`.`message` 
FROM `messages` 
JOIN 
(
    SELECT 
    `from_pic`.`id`, 
    `from_pic`.`profile_pic` 
    FROM `users` AS `from_pic` 
) u1 --- alias 
    ON u1.`id` = `messages`.`from_id` 
JOIN 
(
    SELECT 
    `to_pic`.`id`, 
    `to_pic`.`profile_pic` 
    FROM `users` AS `to_pic` 
) u2 ---- alias 
    ON u2.`id` = `messages`.`to_id` 
WHERE `messages`.`from_id` = 23 
    AND `messages`.`to_id` = 24 
ORDER BY `date_sent` DESC;

这也没有子查询写成：

SELECT 
    `messages`.`from_id`, 
    `messages`.`to_id`, 
    `messages`.`message` 
FROM `messages` 
JOIN `users` AS `from_pic` 
    ON `from_pic` = `messages`.`from_id` 
JOIN `users` AS `to_pic` 
    ON `to_pic`.`id` = `messages`.`to_id` 
WHERE `messages`.`from_id` = 23 
    AND `messages`.`to_id` = 24 
ORDER BY `date_sent` DESC;

来源

2017-05-24 18:04:25 ollie

我明白了。我之前试过这个，但是我得到这个错误，所以我不知道什么是错的：'＃1054 - 未知列'users.profile_pic'in'字段列表' ' – nTuply

@nTuply看到我的编辑，你已经在'users'表的子查询中提供了别名，您需要在那里引用该别名。 – ollie

'＃1054 - 'where子句'中的未知列'from_id'错误很疯狂。你认为我应该发布我的餐桌设计吗？ – nTuply

-1

您需要将您的from_pick和to_pic别名出来，他们不需要在子查询和使用他们因此在你的加入条件，就像这样：

SELECT `messages`.`from_id`, 
    `messages`.`to_id`, 
    `messages`.`message` 
FROM `messages` 
JOIN 
(
    SELECT `users`.`profile_pic` 
    FROM `users` 
    WHERE `users`.`id` = `from_id` 
) AS `from_pic` 
    ON `from_pic`.`id` = `messages`.`from_id` 
JOIN 
(
    SELECT `users`.`profile_pic` 
    FROM `users` 
    WHERE `users`.`id` = `to_id` 
) AS `to_pic` 
    ON `to_pic`.`id` = `messages`.`to_id` 
WHERE `messages`.`from_id` = 23 
    AND `messages`.`to_id` = 24 
ORDER BY `date_sent` DESC;

更重要的是，你甚至不真正需要的子查询：

SELECT `messages`.`from_id`, 
    `messages`.`to_id`, 
    `messages`.`message` 
FROM `messages` 
JOIN `users` AS `from_pic` ON `from_pic`.`id` = `messages`.`from_id` 
JOIN `users` AS `to_pic` ON `to_pic`.`id` = `messages`.`from_id` 
WHERE `messages`.`from_id` = 23 
    AND `messages`.`to_id` = 24 
ORDER BY `date_sent` DESC;

来源

2017-05-24 18:40:25 Uueerdo

哎。如果你打算投票，你至少应该说出答案的问题。 – Uueerdo

MySQL JOIN错误：每个派生表都必须有自己的别名

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