Question

好的，所以我需要编写一个数据库查询，根据key b和key_a的值，从category中获取meta_value。

这是我的sql：

        $cat   = $post['cat'];
        $adh   = $post['adhesion'];
        //get adhesions for category
        $query = $wpdb->get_results(
            "SELECT p.`ID`, pm.`meta_value` FROM {$wpdb->postmeta} pm
            LEFT JOIN {$wpdb->posts} `p` ON `p`.`ID` = pm.`post_id`
            LEFT JOIN {$wpdb->term_relationships} `tr` ON `p`.`ID` = `tr`.`object_id`
            LEFT JOIN {$wpdb->term_taxonomy} `tt` ON `tr`.`term_taxonomy_id` = `tt`.`term_taxonomy_id`
            WHERE `pm`.`meta_key` = 'substrate_box'
            AND `pm`.`meta_key` = 'adhesion_box'
            AND `pm`.`meta_value` = '$adh'
            AND `p`.`post_status` = 'publish'
            AND `p`.`post_type` = 'post'
            AND `tt`.`taxonomy` = 'category'
            AND `tt`.`term_id` = $cat
            ");

我需要做的是为每个包含类别值和第一个元键值（substrate_box）的帖子获取meta_key adhesion_box的值。

这个sql语句返回null，所以我不太清楚怎么做。

Answer 1

将WHERE子句条件移动到ON子句。

SELECT p.`ID`, pm.`meta_value` FROM {$wpdb->postmeta} pm
            LEFT JOIN {$wpdb->posts} `p` ON `p`.`ID` = pm.`post_id`
            LEFT JOIN {$wpdb->term_relationships} `tr` ON `p`.`ID` = `tr`.`object_id`
            LEFT JOIN {$wpdb->term_taxonomy} `tt` ON `tr`.`term_taxonomy_id` = `tt`.`term_taxonomy_id`
            AND  `pm`.`meta_key` = 'substrate_box'
            AND `pm`.`meta_key` = 'adhesion_box'
            AND `pm`.`meta_value` = '$adh'
            AND `p`.`post_status` = 'publish'
            AND `p`.`post_type` = 'post'
            AND `tt`.`taxonomy` = 'category'
            AND `tt`.`term_id` = $cat

Answer 2

这是最终工作的解决方案，原谅其丑陋：

$cat   = $post['cat'];
        $adh   = $post['adhesion'];
        //get adhesions for category
                $query   = $wpdb->get_results("
                                        SELECT pm.`post_id` FROM {$wpdb->postmeta} pm
            LEFT JOIN {$wpdb->posts} `p` ON `p`.`ID` = pm.`post_id`
            LEFT JOIN {$wpdb->term_relationships} `tr` ON `p`.`ID` = `tr`.`object_id`
            LEFT JOIN {$wpdb->term_taxonomy} `tt` ON `tr`.`term_taxonomy_id` = `tt`.`term_taxonomy_id`
            WHERE `pm`.`meta_key` = 'adhesion_box'
            AND `pm`.`meta_value` = '$adh'
            AND `p`.`post_status` = 'publish'
            AND `p`.`post_type` = 'post'
            AND `tt`.`taxonomy` = 'category'
            AND `tt`.`term_id` = $cat
                        ");
    if(is_array($query)){
        foreach($query as $obj){
            $tmps = $wpdb->get_results("
                    SELECT p.`ID` as post_id, pm.`meta_value` FROM {$wpdb->postmeta} pm
                     LEFT JOIN {$wpdb->posts} `p` ON `p`.`ID` = pm.`post_id`
                      WHERE pm.`post_id` = {$obj->post_id}
                          AND pm.`meta_key` = 'substrate_box'
            ");
            if(is_array($tmps) && count($tmps) > 0){
                foreach($tmps as $tmp){
                    $return[$tmp->post_id] = $tmp->meta_value;
                }
            }
        }
    }

这将根据帖子类别和第一个元值选择并返回第二个元值。

按类别和元值获取元值

2 个答案: