内连接mysql php

Question

如何根据用户表中的用户 ID 选择用户名、头像等而不会出现致命错误：请未捕获 PDOException？

PHP：

$req_pm_list = $sql_connection->prepare('SELECT pm.receiver, pm.date_sent, pm.unread, users.username, users.avatar, users.session_id
    FROM pm
    INNER JOIN users AS user_receiver ON pm.receiver = users.id
    INNER JOIN users AS user_sender ON pm.sender = users.id
    WHERE sender = :id OR receiver = :id 
    GROUP BY users.id 
    ORDER BY date_sent ASC');
$req_pm_list->bindValue(':id', $_SESSION['id']);
$req_pm_list->execute();
$pm_list = $req_pm_list->fetchAll(PDO::FETCH_ASSOC);

Answer 1

问题在于，因为您在两个连接中都为 users 表加上了别名，所以在 JOIN 条件、GROUP BY 和 SELECT 列表中对 users 的引用不明确或无效。

您想从SELECT 中选择哪个用户名和头像，发送者还是接收者？ 因为您已经JOINed 两次，使用不同的别名，所以您需要明确您正在使用users 表的哪个副本。

它必须是 user_receiver.username 和 user_receiver.avatar 或 user_sender.username 和 user_sender.avatar，或全部。对 users 的引用此时无效，因为没有加入未别名的 users 表。

同样，users 的别名应该用在JOIN 条件中

我不确定你为什么还要做GROUP BY，但同样的事情发生在那里，users 引用无效，因为连接中没有未别名的users 表，你必须告诉它使用哪个用户表的副本。

查询应该看起来更像这样（我省略了GROUP BY，因为在没有看到一些实际表数据的情况下，不清楚你想用它来完成什么。

SELECT pm.receiver, pm.date_sent, pm.unread, 
    user_receiver.username AS receiver_name, 
    user_receiver.avatar AS receiver_avatar,
    user_receiver.session_id AS receiver_session_id,
    user_sender.username AS sender_name, 
    user_sender.avatar AS sender_avatar,
    user_sender.session_id AS sender_session_id
FROM pm
INNER JOIN users AS user_receiver ON pm.receiver = user_receiver.id
INNER JOIN users AS user_sender ON pm.sender = user_sender.id
WHERE sender = :id OR receiver = :id 
ORDER BY date_sent ASC 

Answer 2

当您为表指定别名时，您不能再引用原始表名，您必须使用别名。所以users.columnName 是错误的，你必须在所有这些地方酌情使用user_receiver.columnName 或user_sender.columnName。

如果您总是想从表中选择与:id 对应的列，可以使用IF 函数。如果您想为其他用户选择列，只需在那些IF 函数中将sender = :id 更改为receiver = :id。

也不需要GROUP BY，你没有聚合任何东西。

SELECT pm.receiver, pm.date_sent, pm.unread, 
    IF(sender = :id, user_sender.username, user_receiver.username) AS username, 
    IF(sender = :id, user_sender.avatar, user_receiver.avatar) AS avatar,
    IF(sender = :id, user_sender.session_id, user_receiver.session_id) AS session_id
FROM pm
INNER JOIN users AS user_receiver ON pm.receiver = user_receiver.id
INNER JOIN users AS user_sender ON pm.sender = user_sender.id
WHERE sender = :id OR receiver = :id 
ORDER BY date_sent ASC