MySQL连接错误：每个派生表都必须有自己的别名

我已经研究了S.O.可能的解决方案，但我的查询仍然无法正常运行

问题是：

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;

表格

用户

id | profile_pic | name

message_id | from_id | to_id | message

表格

消息

id | profile_pic | name

message_id | from_id | to_id | message

---

使用子查询时，需要为其提供别名：

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;

这也可以在没有子查询的情况下编写：

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;

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;

---

使用子查询时，需要为其提供别名：

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;

这也可以在没有子查询的情况下编写：

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;

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;

---

您需要将您的

别名从\u pick

和

移动到\u pic

子查询中不需要的别名，并在联接条件中相应地使用它们，如下所示：

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;

更好的是，您甚至不需要子查询：

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;

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;

---

您需要将您的

别名从\u pick

和

移动到\u pic

子查询中不需要的别名，并在联接条件中相应地使用它们，如下所示：

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;

更好的是，您甚至不需要子查询：

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;

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;

---

我懂了。我以前尝试过这个方法，但是我遇到了这个错误，所以我不确定是什么问题：

#1054-未知列“users.profile_pic”在“field list”

@nTuply查看我的编辑，您在子查询中为

users

表提供了别名，您需要在其中引用该别名。

#1054-where子句中的未知列“from_id”

错误非常严重。你认为我应该发布我的桌子设计吗？@nTuply那可能会有帮助。

用户

是否有

from\u id

列？添加了表。不，

用户

没有

消息

具有

from_id

和

to_id

，我正试图与

用户

加入，以获取我看到的每个用户的个人资料图片。我以前尝试过这个方法，但是我遇到了这个错误，所以我不确定是什么问题：

#1054-未知列“users.profile_pic”在“field list”

@nTuply查看我的编辑，您在子查询中为

users

表提供了别名，您需要在其中引用该别名。

#1054-where子句中的未知列“from_id”

错误非常严重。你认为我应该发布我的桌子设计吗？@nTuply那可能会有帮助。

用户

是否有

from\u id

列？添加了表。不，

用户

没有<代码>消息有

from\u id

和

to\u id

，我正试图加入

用户

以获取每个用户的个人资料图片。如果你要否决投票，你至少应该说答案有什么问题。哎哟。如果你要否决投票，你至少应该说一下答案有什么问题。