[英]Difficulty with join 3 tables in a query in php
我的數據庫有3個表,我希望在select查詢中訪問這些表,但似乎無法正常工作。 從2個表中進行選擇可以很好地工作,所以我知道除了從3個表中進行選擇的代碼以外,其他所有功能都在起作用。 我的數據庫已在PHPmyadmin上創建
表格如下:
forum_replies
forum_topics
用戶
這是我嘗試使用的代碼,並顯示了我希望選擇的字段:
$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);
代碼示例。 謝謝
你錯過了,
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
使用此查詢:
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
除語法錯誤外,您還應使用LEFT JOIN
和表別名。
要同時顯示主題創建者的用戶名,您可以將查詢調整為以下內容:
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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