[英]Difficulty with join 3 tables in a query in php
我的数据库有3个表,我希望在select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,所以我知道除了从3个表中进行选择的代码以外,其他所有功能都在起作用。 我的数据库已在PHPmyadmin上创建
表格如下:
forum_replies
forum_topics
用户
这是我尝试使用的代码,并显示了我希望选择的字段:
$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
你错过了,
以后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
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。
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:
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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