mysql - SQL 左连接没有给我空条目

Question

我正在为 WordPress 制作一个 RSS feed，要求是如果文章设置了促销日期，则应按促销日期排序，否则使用 post_date 否则我来了 执行以下 SQL。

SELECT $wpdb->posts.* from $wpdb->posts 
LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id)
LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) 
LEFT JOIN $wpdb->terms ON ($wpdb->term_taxonomy.term_id = $wpdb->terms.term_id) 
LEFT JOIN $wpdb->postmeta as m2 on ($wpdb->posts.ID=m2.post_id and m2.meta_key='promotion_date') 
where now()>(CASE WHEN m2.meta_value IS NOT NULL 
    then cast(m2.meta_value as DATETIME) 
    ELSE $wpdb->posts.post_date END) 
and $wpdb->posts.post_status='publish' 
and $wpdb->posts.post_type='article' 
and $wpdb->terms.slug = $slug 
and $wpdb->term_taxonomy.taxonomy = 'article_$taxonomy' 
order by CASE WHEN m2.meta_value IS NOT NULL 
      then cast(m2.meta_value as DATETIME) 
      ELSE $wpdb->posts.post_date 
END DESC limit 100;

问题是左连接似乎不会返回没有促销日期的条目。当没有匹配项时，左连接通常会选择第一个表中的所有列，而第二个表中的条目为空。为什么这种情况没有发生？

Answer 1

您在查询 $slug 的“terms”和 WHERE 子句中的“term_taxonomy”时有两个“AND”条件，从而将您的 LEFT-JOIN 转换为 INNER-JOIN。我已将它们移至 LEFT JOIN 组件。否则你应该很好(另外，我只是为表格应用了别名，而不是为了可读性而进行长手写)

SELECT 
      p.* 
   from 
      $wpdb->posts p
         LEFT JOIN $wpdb->term_relationships tr
            ON p.ID = tr.object_id
            LEFT JOIN $wpdb->term_taxonomy tt
               ON tr.term_taxonomy_id = tt.term_taxonomy_id
              and tt.taxonomy = 'article_$taxonomy' 
               LEFT JOIN $wpdb->terms t 
                  ON tt.term_id = t.term_id
                 and t.slug = $slug 
         LEFT JOIN $wpdb->postmeta as m2 
            on p.ID = m2.post_id 
            and m2.meta_key = 'promotion_date'
   where 
          now() > ( CASE WHEN m2.meta_value IS NOT NULL 
                     then cast(m2.meta_value as DATETIME) 
                     ELSE p.post_date END )
      and p.post_status = 'publish' 
      and p.post_type = 'article' 
   order by 
      CASE WHEN m2.meta_value IS NOT NULL 
         then cast(m2.meta_value as DATETIME) 
         ELSE p.post_date END DESC 
   limit 100;

为了查询性能，我建议在帖子表上建立索引(post_status、post_type、post_date)