Difficulty with join 3 tables in a query in php

问题

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

The Tables are as follows:

forum_replies

reply_id
topic_id
user_id
reply_text
reply date

forum_topics

topic_id
category_id
user_id
topic_title
topic_description
topic_date

users

user_id
username

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);

Example in code would be appreciated. Thanks

回答1:

Use this query:

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

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

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

回答2:

You miss the , after users.username..

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

来源：https://stackoverflow.com/questions/29812596/difficulty-with-join-3-tables-in-a-query-in-php

标签

php

sql

join

forum