在复杂的多表查询上进行MySQL分页

Question

目前，我正在运行这样的MySQL查询

$query = "
        SELECT 
        t1.id AS 'post_id',
        t1.post_title AS 'post_title',
        t1.post_status AS 'post_status',
        COUNT( t1.id ) AS 'row_count', //this one is not working
        COUNT( t2.tag_id ) AS 'link_count',
        GROUP_CONCAT( t2.tag_id) AS 'link_id',
        GROUP_CONCAT( t2.url) AS 'url',
        GROUP_CONCAT( t2.title) AS 'title',
        GROUP_CONCAT( t2.active) AS 'active',
        FROM $posts_table AS t1
        JOIN $tags_table AS t2 ON ( t1.id = t2.post_id )
        GROUP BY t1.id
        ORDER BY t1.id DESC
        LIMIT $start_from, $row_to_show;
    ";

除了COUNT( t1.id ) AS 'row_count',行之外，此查询的每个部分都执行良好。

我想计算在t1表中找到的行总数，但是当我转储数组时

$result = $wpdb->get_results($query);
var_dump($result->row_count);

它给了我NULL 。

如果我在循环中使用它

foreach ( $result as $result ){
    echo $result->row_count;
}

然后，该值与在下一行COUNT( t2.link_id ) AS 'link_count',声明的link_count相同COUNT( t2.link_id ) AS 'link_count',给出我想要的值。

请给我一些关于如何在此查询上使用分页（例如30个结果中的10个）的想法。

Answer 1

我认为问题出在GROUP BY。

试试这个查询：

$query = "
        SELECT 
        t1.id AS 'post_id',
        t1.post_title AS 'post_title',
        t1.post_status AS 'post_status',
        COUNT( t1.id ) AS 'row_count', 
        COUNT( t2.tag_id ) AS 'link_count',
        GROUP_CONCAT( t2.tag_id) AS 'link_id',
        GROUP_CONCAT( t2.url) AS 'url',
        GROUP_CONCAT( t2.title) AS 'title',
        GROUP_CONCAT( t2.active) AS 'active',
        FROM $posts_table AS t1, $tags_table AS t2 
        GROUP BY t1.id 
        HAVING t1.id = t2.post_id 
        ORDER BY t1.id DESC
        LIMIT $start_from, $row_to_show;
    ";

Answer 2

像COUNT( DISTINCT X ) .. GROUP BY X类的问题。GROUP COUNT( DISTINCT X ) .. GROUP BY X将始终得出值1，如果跳过DISTINCT，您将获得组合联接的数量

有多种方法可以在mysql中获取计数，但仅在php中使用mysql_num_rows()或count()轻松实现。

$results = $wpdb->get_results($query);

foreach($results as $result)
{
    $result->row_count =  count($results);
    ...
}

上面仅显示获取的行数，如果需要总计，则需要使用SQL_CALC_FOUND_ROWS和mysql_num_rows()

$query = "SELECT SQL_CALC_FOUND_ROWS ...

和

$result_count = mysql_num_rows();

Answer 3

我尝试了很多，但没有成功，因此我仅针对分页进行了另一个单独的数据库查询。

$page_query = "
    SELECT p.id 
    FROM 
    $posts_table as p, 
    $tag_table as c
    WHERE p.id=c.post_id
    GROUP BY p.id
";

它给了我可用于分页的总行数的结果。