让 JOIN 工作的问题

Posted 2023-05-09

【中文标题】让 JOIN 工作的问题

【英文标题】：Issue getting JOIN to work

【发布时间】：2015-08-18 06:01:43

【问题描述】：

我有以下查询

$stmt2 = $con->prepare("SELECT `id`, `category_id`, `topic_id`, `post_creator`, `post_content`, `post_date` FROM forum_posts WHERE `category_id`=? AND `topic_id`=?");

我现在正在尝试加入我的用户表，以便将forum_posts.post_creator 与users.id 匹配并从中获取用户名。

我正在尝试这个，但查询一直失败..

$stmt2 = $con->prepare("SELECT forum_posts.id, forum_posts.category_id, forum_posts.topic_id, forum_posts.post_creator, forum_posts.post_content, forum_postspost_date FROM forum_posts
        WHERE forum_posts.category_id=? AND forum_posts.topic_id=?
        INNER JOIN users
        ON forum_posts.post_creator = users.id");
       if ( !$stmt2 || $con->error ) 
        // Check Errors for prepare
         die('Stmt2 SELECT prepare() failed: ' . htmlspecialchars($con->error));
        

            $stmt2->bind_param("ii", $cid, $tid);
            if(!$stmt2->execute()) 
                die('Stmt2 SELECT execute() failed: ' . htmlspecialchars($stmt2->error));
            
            $stmt2->store_result();
            $stmt2->bind_result($post_id, $post_category_id, $post_topic_id, $post_creator, $post_content, $post_date, $posts_username);

我收到此错误，但无法弄清楚该部分有什么问题。

Stmt2 SELECT prepare() failed: You have an error in your SQL syntax; check the manual that corresponds to your mysql server version for the right syntax to use near 'INNER JOIN users ON forum_posts.post_creator = users.id' at line 3

我做错了什么？

【参考方案1】：

inner join 是from 子句的一部分，而不是where 子句的一部分：

SELECT forum_posts.id, forum_posts.category_id, forum_posts.topic_id, forum_posts.post_creator, forum_posts.post_content, forum_postspost_date 
FROM forum_posts
        INNER JOIN users
        ON forum_posts.post_creator = users.id        
WHERE forum_posts.category_id=? AND forum_posts.topic_id=?

【讨论】：

我应该忽略这部分吗？ users.username

@Paul 在您的问题中找不到这部分

对不起.. 我会这样做来获取用户名吗？ SELECT forum_posts.id, forum_posts.category_id, forum_posts.topic_id, forum_posts.post_creator, forum_posts.post_content, forum_postspost_date, users.username

是的。你可以用它来获取username

效果很好！谢谢！

【参考方案2】：

你准备好的陈述应该是这样的。 JOIN 应该在 Where 之前

("SELECT forum_posts.id, forum_posts.category_id, forum_posts.topic_id,
         forum_posts.post_creator, forum_posts.post_content, forum_postspost_date 
         FROM forum_posts INNER JOIN users 
                          ON forum_posts.post_creator = users.id
                         WHERE forum_posts.category_id=? AND 
                               forum_posts.topic_id=?"   
    );