在PHP查询中难以连接3个表 [英] Difficulty with join 3 tables in a query in php

问题描述

我的数据库有3个表，我希望在选择查询中访问这些表，但似乎无法正常工作.从2个表中进行选择可以很好地工作，因此我知道除了从3个表中进行选择的代码之外，其他所有功能都在起作用.我的数据库已在PHPmyadmin上创建

My database has 3 tables i wish to access in the select query but I cannot seem to get it to work. Selecting from 2 tables works fine so I know everything else is working apart from my code for selecting from 3 tables. My database has been created on PHPmyadmin

表如下:

forum_replies

forum_replies

• reply_id
• topic_id
• user_id
• reply_text
• 回复日期

forum_topics

forum_topics

• topic_id
• category_id
• user_id
• topic_title
• topic_description
• topic_date

用户

• user_id
• 用户名

这是我尝试使用的代码，并显示了我希望选择的字段:

This is the code I have tried to use and shows the fields I wish to select:

    $queryreply = "SELECT forum_replies.reply_id, forum_replies.topic_id, forum_replies.user_id,
                       forum_replies.reply_text, forum_replies.reply_date, users.user_id, users.username
                       forum_topics.topic_id,forum_topics.topic_title, forum_topics.topic_date
                       FROM forum_replies
                       JOIN forum_topics
                       ON forum_replies.topic_id = forum_topics.topic_id
                       JOIN users
                       ON forum_replies.user_id = users.user_id

                       ";


        $result = mysql_query($queryreply) or die (mysql_error());
        $row = mysql_fetch_array($result); 

代码示例.谢谢

推荐答案

使用此查询:

SELECT a.reply_text, a.reply_date, b.topic_title, c.username
FROM forum_replies a
LEFT JOIN forum_topics b ON a.topic_id=b.topic_id
LEFT JOIN users c ON a.user_id=c.user_id
// apply WHERE, ORDER, GROUP if needed

除语法错误外，您还应使用LEFT JOIN和表别名.

Apart from syntax errors, you should use LEFT JOIN and table alias in your case.

要同时显示主题创建者的用户名，您可以将查询调整为以下内容:

To show also the topic creator's username, you can adjust the query to the following:

SELECT a.reply_text, a.reply_date, b.topic_title, c.username AS reply_user, (SELECT username FROM users WHERE user_id=b.user_id) AS topic_creator
FROM forum_replies a
LEFT JOIN forum_topics b ON a.topic_id=b.topic_id
LEFT JOIN users c ON a.user_id=c.user_id
// apply WHERE, ORDER, GROUP if needed

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