是的，我对此并不特别熟练，这实际上是我的第一个连接查询，所以请保持温和。我将尽可能多地提供详细信息，因为它可能会像煎锅一样砸在你们大多数人的脸上，但它让我抓狂!

我在查询我试图用 codeigniter 编写的博客时遇到问题。我已经设置了一个查询，其中包含 2 个帖子连接，它的类别使用三个表:帖子、类别和 posts_categories 现在我也尝试连接我的评论表，以进行计数。

这是我的模型中的代码，它显示了我编写的两个通用帖子:

            $this->db->select('posts.id,
                            posts.title,
                            posts.slug,
                            posts.content,
                            posts.author,
                            posts.date,
                            posts.time,
                            posts.tags,
                            posts.status,
                            GROUP_CONCAT(categories.name SEPARATOR \'-\') AS categories
                            ');
        $this->db->group_by(array('posts.id'));
        $this->db->from('posts');
        $this->db->join('posts_categories', 'posts_categories.blog_entry_id = posts.id');
        $this->db->join('categories', 'posts_categories.blog_category_id = categories.category_id');
        $query = $this->db->get();
        return $query->result_array();

这是结果:

(
[0] => Array
    (
        [id] => 1
        [title] => My first blog post!
        [slug] => my-first-blog-post
        [content] => This is my first blog post. Don't worry, it's just a test, my real blog won't be this boring, hopefully!
        [author] => Joni
        [date] => 2012-01-23
        [time] => 00:00:00
        [tags] => Testing
        [status] => 
        [categories] => Testing-More Tests-Test
    )

[1] => Array
    (
        [id] => 2
        [title] => This is another test-post
        [slug] => this-is-another-test-post
        [content] => Well you guessed it. another boring test post, enjoy!
        [author] => Joni
        [date] => 2012-01-23
        [time] => 00:00:00
        [tags] => Sexy
        [status] => 
        [categories] => Test
    )

)

现在，当我修改查询以实现第三个评论连接时，如下所示:

            $this->db->select('posts.id,
                            posts.title,
                            posts.slug,
                            posts.content,
                            posts.author,
                            posts.date,
                            posts.time,
                            posts.tags,
                            posts.status,
                            GROUP_CONCAT(categories.name SEPARATOR \'-\') AS categories,
                            count(comments.id) as total_comments
                            ');
        $this->db->group_by(array('posts.id'));
        $this->db->from('posts');
        $this->db->join('posts_categories', 'posts_categories.blog_entry_id = posts.id');
        $this->db->join('categories', 'posts_categories.blog_category_id = categories.category_id');
        $this->db->join('comments', 'comments.post_id = posts.id');
        $query = $this->db->get();
        return $query->result_array();

我最终得到了这个

(
[0] => Array
    (
        [id] => 1
        [title] => My first blog post!
        [slug] => my-first-blog-post
        [content] => This is my first blog post. Don't worry, it's just a test, my real blog won't be this boring, hopefully!
        [author] => Joni
        [date] => 2012-01-23
        [time] => 00:00:00
        [tags] => Testing
        [status] => 
        [categories] => Testing-More Tests-Test
        [total_comments] => 3
    )

)

如果您已经走到这一步，很抱歉拖了这么久，在此先说声谢谢!

干杯乔尼

最佳答案

您需要使用 LEFT OUTER JOIN 否则您只会得到有评论的帖子。当您执行 INNER JOIN(默认)时，它将要求左侧的任何内容在连接的右侧都有一个匹配元素。如果在右侧找不到匹配项，则会将其忽略。 LEFT OUTER JOIN 将保留联接左侧的所有元素，无论右侧是否存在匹配项。

改变这个:

$this->db->join('comments', 'comments.post_id = posts.id'); 

到

$this->db->join('comments', 'comments.post_id = posts.id', 'left outer' );

关于php - 将另一个数据库连接添加到我的查询导致一些行看不到，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9933776/